CRAP is a previous, lengthy, set of posts (entitled "ChatRoom Anxiety & Paranoia (CRAP)" It might be a good read....
However, the majority of it relates to irrational parental paranoia regarding the internet, chatrooms and perverts. Not really prevalent to this situation.
Here is my advice.
Don't ban or forbid the computer OR the net... that will just make her resentful, and probably even more eager to do it (and as you said, she can likely access it at school)
More importantly... has she gone to any sort of counselling? If not, do you think she might want to? If so
The thing to do that is better than doing nothing (and better than doing too much) is discussion. While sexual abuse is a terrible thing, that does not, ipso facto, make what she's been doing a bad sign. Talk to her, find out how she feels about it.
Again, good luck.