I am trying to find the ruling for the following conundrum.

School of evocation at level 14 get the ability to overchannel. Basically what it does is for any damage spell cast between level 1-5 it will do maximum damage. "When you cast a wizard spell of 1st through 5th level that deals damage, you can deal maximum damage with that spell.".

Now, for the rule interpretation.

What happens IF you cast a level 5 spell, BUT at a higher level? Take the 5th level spell "Flamestrike" for example. If the creature fails a save it takes 4d6 fire and 4d6 radiant damage. If you cast this at 9th level then you would add 4d6 radiant and 4d6 fire damage, a total of 8d6 radiant damage and 8d6 fire damage. The actual spell is a 5th level spell, it's just that it's cast at a higher level.

With the overchannel ability this would mean that the creature takes 48 radiant damage and 48 fire damage, or half at a save. If the creature fails we are looking at 96 damage.

Is there a rule that covers this, or will it be more of a house rule for each DM to make?

The PHB (pg201) about spellcasting at higher level says:
"When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell assumes the higher level for that casting."

So no Overchannel wouldn't work with a level 6+ Flamestrike because it's considered in that instance a level 6+ spell.

Edit:
Sage Advice agrees: https://www.sageadvice.eu/2015/05/15/overchannel-slot-higher-slot/