Ditto GTs comments - Would not bit again till mouth no longer sore and then would make sure bit was fitted correctly, was the right size and ensure bit suitable for horse and rider to ensure mouth didn't get sore again.
Why did your instructor put the bit up 2 holes? Seems a bit drastic...one perhaps...but 2? I would leave your horse until the mouth is better and then put the bit in the way you had it before the instructor messed with it...assuming it fitted correctly like that.
My mare had a rubbed mouth that just wouldn't heal (serves me right for letting someone else ride her!
It was such a nightmare - opened up at any opportunity - I used sudocrem (thoroughly cleaned it after riding and then applied the sudocrem) However prior to riding I put germolene second-skin on the area - then lathered it in vasalene to protect it whilst riding - Unfortunately it wasn't an option for her not to be worked and having tried a bitless bridle it was safer to use the snaffle!!
I've found the best thing to use on my event horse last year when his mouth was split was alva miracle cream that I found on their stand at Burghley. Put on 4 - 5 times a day and massage well in then put vaseline on too if you have to ride with a bit.