I suppose I am quite lucky with my School Admins, I get on well with them and have my own Admin account anyway. The students at my school even have our own servers now amoungst other things.
I think you should either forget the flaw or let the admins know, definitely don't do anything. Because if you do do something then you could ruin a lot of peoples work, possibly coursework which could then affect the whole of their lives. Its just not worth it. And finding the hole is always more fun than when you have got it.