What your teacher probably did was take a pair of pliers and press down on (squeeze) the wire nearest the end you put in your mouth (first wire). This closes the reed a little bit and makes high notes a little easier to play. The down side is that sometimes if it's done too much the reed closes up on you and you can't play some other notes. You can always reverse the process (take a pair of pliers and VERY GENTLY squeeze on the same wire but on the sides this time, to open the reed back up again). If you do it too much, just squeeze the top and bottom rather than the sides to close it back up a tiny bit again. You can play with this for hours - until you break the wires and have to rewrap with another wire!

ok, no. my reed was closed, and she opened it up a little(no pliers, just squeezing). what it looked like she was doing was something with a nail file. i kept the same embouchure on my reed before, but it seemed a little...it kept resistance enought against my lips so my teeth didn't press in too much. the reed now is very...its weaker, so there's little resistance.

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by the way, my high notes were perfectly fine until just today. my teacher said my reed was unbalanced (because it was store bought) so she was trying to balance it out.

Yeh, a lot of store bought reeds can be unbalanced. She probably did have a small file, I presume she was sanding down various parts of the reed because every part of the reed affects something. They're much more complicated things than people realise, i think.
Next time your teacher adjusts your reed for you and you don't understand what she's doing, you should ask her. I'm sure she'd be happy to explain, and you learn something useful.

One way I can normally tell if my reed is dying is if the E above open (whisper key) F drops in pitch. Also my Db becomes very unstable.

I agree about the upper Db (C#) becoming unstable. When this gets a lot softer, or harder to play properly, I found it usually means the reed isn't supporting it like it should, and that's the beginning of the end.

Sorry for sounding stupid, but I'm guessing that if the reed cracks, its done?

Not necessarily. I've told this story before but it will bear telling again. A long time ago, when I was in hi school and had solo contest coming up, I had one reed, no money, and the reed was so hard it was horrible to play. I didn't know how to fix reeds back then and when the time came for our band director to sit in on our solo practice and see how we were doing, he could hardly believe how bad I was. He chewed me out because he figured I hadn't practiced - it wasn't that; I just couldn't do anything with that hard reed. Well, I left the band room and was nearly in tears, and was so frustrated at what I figured was the injustice of it all, that I took my reed in my mouth and bit down hard on it. To my horror, it cracked all the way from top to bottom. I was absolutely transfixed with fright; I had no money for another reed. So I played on that reed the next day, and guess what? It was perfect, I played beautifully, got a first on my solo, and subsequently spoiled the moment by saying "Nyah, nyah" to my band director (who didn't appreciate the sentiment at all. I got another chewing out).

Sorry, I've been away ages, but i'm here to catch up now!
I've always found that Db is a dodgy note. You just need plenty of support for it and a good ear to keep it in tune. Though a duff reed wouldn't help.
I think it depends how big the crack is, and where abouts it is. I've had the corners of a reed crack off completely and it's still worked exactly the same. and little cracks probably don't make too much difference. But Huge cracks all the way down the blade usually make the reed a write-off. Drew was just incredibly lucky.
Also, just for the record... *slaps Drew on wrist for chewing his reed* :p