myoasis - i had these often too. i used to *obsess* about some of my teachers, and still did, even after i'd left high school and lost touch with them. now i'm at uni so of course i crush over my tutors instead

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i'm 24 now so it's been a while since high school, but i think i've come to realise that a lot of my obsession stemmed from some of the things other people here have mentioned - feeling a bit special, listened to or cared for, even if the teacher didnt know you very well. for me - teachers were the only safe and stable adult influences in my life at that time. they taught me stuff i was interested in, but they didnt get angry or beat me up if i didnt understand straight away.
so i guess it's no wonder that i used to idolise them.
one thing i've found - if you get one of those crazy, heady crushes about someone you don't know that well - the easiest way to quash them is to figure out what it is that attracts you to that person. putting a name to it almost always takes the edge off, and then you're free to appreciate the real relationship for what it is.
it's also so, so useful in finding out what you want from future relationships. and unless you have err, weird kinks you haven't told us about - those traits are pretty easy to find. for me, it would be - someone who is safe, who will listen to me/make me feel special, is committed to their job (i have no time for people who hate the job they're in), etc etc etc.
instead of making it this magical quality - break it down, and you'll have a lot more power.