When I was 11 or 12, I borrowed a small amount from a teacher at my school during a trip and I cannot remember whether I repaid him back or not. I am too embarrassed to approach and ask him about this so I don't know what I should do. Am I condemned to hell, until this is repaid?
Go and ask! The main sin is if you don't take the shame of asking. This is serious because there is no logical way to get out from this sin. Asking, however, may save your soul.
I would suggest you go up to the teacher and talk about it. Asking is the only way out. All you need to do is simply go up to him and ask him the same question you have asked us. Worst case scenario, he will smile and ask you to payup, best case you have already re-paid or he will excuse you from repaying.
Either way, ask.
And Allah Knows Best.