One of the summer school classes I have is for D&D (my school has some very fun extra classes) and the bard I'm playing as learned polymorph. The DM at the table I play at says I can't use it on myself, there's no mention in the book of using it on yourself, but the teacher of the class (I think he's been playing for longer than the other DM's) said that I probably could, as long as the creature I was gonna turn into (it was a young brass dragon) had 8+ in wisdom of intelligence. Can someone tell me whether my bard can use it on himself or not?

You can use Polymorph on yourself, as you are a creature you can see within range. But you cannot turn yourself into a dragon, as you can only turn into Beasts using the spell. 

The new form can be any beast whose challenge rating is equal to or less than the target's (or the target's level, if it doesn't have a challenge rating). The target's game statistics, including mental ability scores, are replaced by the statistics of the chosen beast. It retains its alignment and personality.

You keep your personality but lose your mental statistics when you do this though, so you need to be cautious. A low intelligence beast may not be able to enact the complex plan you have in mind. ;)

Note: The spell True Polymorph allows you to choose from more than just beasts.

unlike previous editions, there is no stat requirement for casting spells, though anything with an intelligence 3 or less is "animal intelligence" and can't communicate with words, perform complex actions, or be concerned with stuff beyond basic survival though.

Polymorph is usually (but not exclusively) used either as an offensive spell to temporarily remove a high threat enemy or as a stealth spell to smuggle people in where they shouldn't be.

As Mellie said, you (the caster) are a creature, and people can see themselves unless blind, invisible, or some other shenanigans are going on; therefore, you are a valid target.

I also thought that I could use polymorph on myself because I can see myself within range, but it's a concentration spell, and that's why the teacher said 8+ intelligence or wisdom.

I'm not aware of any rule that specifies a minimum intelligence or wisdom to maintain concentration. The rules on concentration don't mention anything about that.

Even the Feeblemind spell doesn't say it breaks concentration - it just says you can't cast any new spells - and it drops your intelligence to 1.

Also bear in mind polymorph requires you to pick a beast with a *lower* cr rating than the targets cr rating or level. So you cant right off the bat turn into a cr 14 creature. 

Quote from Desdemona_XI >>

Also bear in mind polymorph requires you to pick a beast with a *lower* cr rating than the targets cr rating or level. So you cant right off the bat turn into a cr 14 creature. 

 
Lower than or equal to.

A Level 4 character can be polymorphed to a CR 4 or lower beast.

Right. Sorry meant to state it that way. Point being even if a dragon were an option which it isnt being a dragon not a beast. Its CR would almost certainly hamstring the idea anyway.

Of you are attempting to use the spell offensively, consider casting it on another PC or friendly NPC to turn them into an elephant or great ape. If you turn yourself into the biggest threat the incoming repeated attacks are likely to cause you to lose concentration, where if cast on a friendly damage or direct attacks will not break the spell.

Quote from Desdemona_XI >>

Also bear in mind polymorph requires you to pick a beast with a *lower* cr rating than the targets cr rating or level. So you cant right off the bat turn into a cr 14 creature. 

Quote from Desdemona_XI >>

Right. Sorry meant to state it that way. Point being even if a dragon were an option which it isnt being a dragon not a beast. Its CR would almost certainly hamstring the idea anyway.

Note that the OP said a Young Brass Dragon, which is CR 6.  Bards can cast Polymorph starting at level 7, so the CR is not a problem.  The creature type ("dragon", when the spell requires "beast") is, on the other hand.

Does anyone else have a problem with the wizard turning into a beast with the same CR as the wizards level?  A level 7 wizard turning into a CR 7 creature seems a little ridiculous. A CR rating is supposed to mean an equal challenge for a party of four of that level.  So the L7 wizard in say a party of 5 changes into a CR7 creature that is as powerful as the other four members of his party combined!

not really an issue - since it is a 4th level spell and the ability for either the magic to be dispelled or the wizard to drop con after 1 round.

Not to mention that if a Wizard is polymorphing into a beast, they can't cast anymore spells until they leave that form.

A big deal considering that casting spells in the Wizard's bread and butter. 

Quote from VAPypr >>

Does anyone else have a problem with the wizard turning into a beast with the same CR as the wizards level?  A level 7 wizard turning into a CR 7 creature seems a little ridiculous. A CR rating is supposed to mean an equal challenge for a party of four of that level.  So the L7 wizard in say a party of 5 changes into a CR7 creature that is as powerful as the other four members of his party combined!

 Nah. The biggest threat will always be the dread thread necromancer.

I know this an old post. But I have a question about cr and polymorph. When a player casts polymorph on themselves or another player is it based on their level as they do not have a CR. So at level 7 I could become a Giant Shark CR5 or at level 8 become a T-Rex?

Quote from Naihiri >>

When a player casts polymorph on themselves or another player is it based on their level as they do not have a CR.

Yes.

The new form can be any beast whose challenge rating is equal to or less than the target's (or the target's level, if it doesn't have a challenge rating).

Quote from Naihiri >>

I know this an old post. But I have a question about cr and polymorph. When a player casts polymorph on themselves or another player is it based on their level as they do not have a CR. So at level 7 I could become a Giant Shark CR5 or at level 8 become a T-Rex?

 Yup. One of the favorite uses for it in our party is a giant ape (King Kong, more or less). When the caster doesn't cast it on themself, you tend to get a lot more rounds out of it.

I see people mentioning on this topic about concentration, but I don't see anything about concentration for Poylmorph.  It lasts until 1 hour has passed or until you get reduced to 0 HP.  You can't cancel it early either as far as I can tell, once it cast you live with the consequences. 

Quote from Bursur >>

I see people mentioning on this topic about concentration, but I don't see anything about concentration for Poylmorph.  It lasts until 1 hour has passed or until you get reduced to 0 HP.  You can't cancel it early either as far as I can tell, once it cast you live with the consequences. 

Polymorph:

Duration: <C> 1 Hour

The <C> means concentration.