Creating a false Dragon encounter for level 3 players

So I'm new to DM and feel I have a pretty compelling first encounter to learn the ropes with.  Long story short a magic amulet is stolen by some rogue apprentice wizards and the players are tasked with returning it to the school from which it came.  The rogue wizards believe it can be used to bind a young dragon and attempt to do so.  They quickly lose control and the amulet actually causes the dragon to grow to a size thats large even for a fully grown dragon and not bind it.  

This is where my question comes in.  The players will already encounter a powerful (to them) boss wizard in the final room and uncover a hidden room.  This is the room the dragon will be.  I'm not sure how to run this part of the encounter though.  I want them to be able to defeat the dragon by removing/ destroying the amulet at which point the dragon shrinks down to a very small size, like that of a small dog.  completely harmless at that point.  However, if they try to fight it in it's current form I'm almost positive they will be wiped.  How would you navigate this?  I don't want to make it overly obvious they remove the amulet, but I also want them to figure it out.  At the same time I don't want the dragon to straight up slaughter them.  Any ideas?  brainstorming?  

EDIT:

Woah woah woah, this is your planned FIRST encounter, so players can learn the ropes? I would recommend starting them off with an actual low-level encounter to learn the ropes. Show them an adult dragon and they might think you're trying to kill them.

The idea is good, but not for the very first fight. Ease players into it first.

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When describing things, our subconscious pays the most attention to the first and last things you describe. Describe the amulet glowing brightly while the transformation is in effect. Calling attention to it is a clue. If the players are slow to recognize the clue, you can repeat it or mention it again alongside other information. Also, you can tell the players that this shouldn't be treated like a straight-up fight by saying "You all recognize that this foe is beyond your ability to defeat with force of arms."

As for not killing he players, give the dragon the HP of a fully grown adult, but because it's not actually an adult, don't give it all the strength and abilities of one. Basically, it has the HP of an adult, but the attack power of a very young dragon. At level 3, that should be enough for the dragon to win by attrition.

A fully grown dragon is gargantuan in size. So you want something bigger than that? 

Maybe instead of a dragon, per se, it’s a drake. Most of them are medium, so you can size it up to large, and it will be of a CR appropriate for the party level. 

You still talk it up like it’s going to be this gigantic dragon, but turns out those rogue wizards just didn’t know what they wee talking about. Or they were embarrassed that a drake got the better of them and they’d inflated it in their minds.

A blue dragon wyrmling is CR 3.

 Making a creature grow larger does not affect it's hitpoints, etc, normally.

All you need to do is to create the atmosphere of oh my god it's an adult dragon!  rather than actually give it massive powers.

Maybe rule the amulet also gives it some temp hitpoints and or some illusory damage.

ok ok ok, maybe you guys are right that it's a bit much.  I guess I really like the idea if it could play out the way I want it to.  I also don't want to railroad the party which I think is where I'm getting hung up.  The idea is for them to not actually fight the dragon but to disarm the amulet which essentially defeats the dragon.  

Quote from johnedlin8 >>

ok ok ok, maybe you guys are right that it's a bit much.  I guess I really like the idea if it could play out the way I want it to.  I also don't want to railroad the party which I think is where I'm getting hung up.  The idea is for them to not actually fight the dragon but to disarm the amulet which essentially defeats the dragon.  

 Well, one problem is the rules for removing a worn item are a bit lax. There’s no called shots, so it’s tough to go. And if you introduce a way to do it, they expect to be able to in every fight. 
Also, players rarely realize this kind of thing. Mostly it’s just try to smash the bag of hit points In front of them. Unless you really, really telegraph that they can remove it for an easier fight. But then you’re still back to the first issue.

yeah that's what I was worried about.  I figured they would try to go toe to toe and unless I made it mind numbingly obvious.  This is why I came here in case somebody has done something at all similar and had a decent solution.  I figured the rule for removing the amulet can be bent to my will being the DM.  Doing so without breaking the emersion by making it stupidly obvious was my bigger concern.  I might need to get creative on how I can keep my concept, while ditching the actual combat attached to the dragon.  Maybe focus more on rp dialogue or something instead.  Especially since they already fought the actual boss in the room prior to the dragon engagement, so they won't be lacking action.  If I come up with any new ideas i'll post here and see what ya'll think.  I appreciate the feedback

If you want to teach your players the game, start with a starter. I did, and I know a lot of the DMs on this forum did. They're all excellent and teach the rules well. Try one out, and then you can move on to your own creations. It will make the experience more enjoyable for everyone. I recommend the Lost Mines of Phandelver, although there is also Dragons of Stormwreck Isle, Dragon of Icespire Peak, and Heroes of the Borderlands.

You're facing the problem that many DM's fall prey to - namely that you are attempting to write a story, and then have the players reveal it. You've rightly noted that this would involve railroading.

What you need to aim for is foreshadowing, foreshadowing, and more foreshadowing, to try and give the party the tools you have made for the fight, without writing it so that they have no choice but to use it.

I'll take an example from my campaign, where the party inadvertently drew a blue dragon back to town. They were level 9, I believe, so it was within their capabilities to defeat. However, I foreshadowed for them, right before they even encountered this dragon, that the town had been mis-sold a weapon called the Hashbow, which is now on every roof, by a pushy salesgnome who said it could defend against anything, land or air. This same gnome also made an automated slaughterhouse using iron golems to speed up the local cattle drive (this was the original plot which brought the party into the game). I also foreshadowed that the buildings were interlinked with new copper piping, which led to a huge water tower on the mountain slopes. I also had the cows under a curse (which is why they stole from the dragon) wherein if they were damaged, they duplicated. This was itself foreshadowed with the predatory animals they found having burst open (if they took a bite, the bite grew back into a cow!).

I had planned for the party to use the Hashbows, and to potentially either lure the dragon into the slaughterhouse for the Golems to attack it, or to trick the dragon into taking a bite out of one of the cows. I also foreshadowed in the dragons lair - a silver mine - that its breath was conducted by the metals, which would also happen with the copper pipes. They may also try to drop the water tower onto the dragon. I gave them options, and planned for them all, but didn't hang the plot on any specifically. They had to defeat a dragon - I just gave them extra tools.

Well, when the party finally got back with the dragon chasing, maybe 4 sessions later. They made a beeline for the Hashbows, and they underperformed. The barbarian set to try and bring down the water tower, but didn't plan how to get the dragon under it. The dragon fought back a lot, and then the warlock used some magic items I had put in the lair to teleport the dragon to the slaughterhouse, where it was mostly killed, before it escaped for the party to finish it off. In so doing, it broke the (foreshadowed) wards on the slaughterhouse which kept the golems contained, presenting a new threat to the battle, which they managed to re-contain.

The story which happened was so, so much better than anything I could have written alone. I gave them a problem - "a dragon is attacking the town" and they worked with the tools I'd given them to solve it.

For your scene, I would foreshadow the idea of removing a powerful item from someone to disempower them. I'd give them some tools to restrain the dragon, like a chandelier above suspended on numerous chains. Give them the tools, but don't tell them to use them, and the idea coming from them will make the fight all the better!