Does Great Teacher's effect count towards the attack you just played? Cause if I play an attack and then use Great Teacher's E, the next attack, if it's, say, 5 difficulty, would it be a 6 or a 5?

Yes it counts the attack you are enhancing.

So say you play an attack (which is a reversal), enhance w/ GT.

You now have 2 cards in your card pool: your attack and GT.

The difficulty of your next attack will be -1, however you will have +2 from progressive difficulty. Seems kind of pointless, except your third attack will be -2 difficulty and +3 progressive.

Note that attacks aren't added to your card pool until they're successfully played, so attacks won't count them selves to reduce difficulty.