What do you have to do when your social security number is discovered by someone?

I am in a pretty bad situation. About a week ago, in class, I was returning the required materials that I received in the starting of the quarter. They signed a check-out slip that we got in the starting of the quarter confirming that we returned everything that we needed to return (and so we could keep it for our records). This slip had my full name, signature, and social security number on it. The problem is, I forgot to put it in my backpack when I left, and when I came back to see if it was still there, it was gone. I know the staff or teachers didn't take it because they would have informed me or the staff would have informed the teachers and they would have given it to me (I even talked to the teachers about it and they let me check my locker that I used to see if I accidently put it in there, but I didn't find it). The problem is that someone, or more than one person, most likely already saw it and could have just copied it down and thrown in away; getting the checkout slip back is totally irrelevant.

Regardless of whether it was taken by someone to use for malicious purposes or not, I have to assume that it was because my credit, amongst other things, could possibly be destroyed if I become passive about this. My question is, what should I do right now? I read about this online, and apparently the law can't help you if you only have an assumption that someone might be using it for malicious purposes or if you lost it in a situation where someone would have access to it (though you can contact certain companies that deal with this stuff).

Also, at this point (a week after it went missing), assuming that they or someone they gave it to has the knowledge of how to use it for malicious purposes, what are they capable of doing in the short term and in the long run (because I'm sure they will be able to do things to me for the rest of my life, since your social security number stays with you for the rest of your life). I already know that some people in the class have knowledge about things like hacking and identity theft (which makes me more nervous), although they wouldn't admit it to anyone they don't know. The quarter is already over, if that matters.

Have any of you been in this situation? How screwed am I?

I have read about who to contact about things like this, but at the moment (though this may sound strange), I would rather get some personal opinions before going straight to professionals.

I would greatly appreciate any help that any of you can give me.

Jam it back in, in the dark.

Sorry if I'm sounding excessively paranoid, but there is one more thing I forgot to mention- it also has my home phone number on it (this might be significant because you can use certain websites to find information on someone, like their address, through their phone number). How much of an impact does the home phone number add?

There's nowhere I can't reach.