I Google weird things when looking for specific types of people to interview.
I just tried "shooting at my school" and found a number of students talking about a shooting at Dawson College (Montreal):
http://www.newgrounds.com/bbs/topic.php?id=568994
http://musicals.net/forums/viewtopic.php?t=51908&start=0
http://www.ubpn.org/messageboard/thread.jspa?threadID=15192&messageID=120677
http://jesterz-dance.deviantart.com/
Here's one who doesn't say where she's from:
http://forums.plentyoffish.com/datingPosts5336334.aspx
This one's also about Dawson, but a few posts down, someone says there was a shooting at his school in 1997:
http://forum.newsarama.com/showthread.php?t=83907
(I stopped on page 2.)
If it were me, I'd try Googling lots of phrases that a teen might type if they'd just been in a situation like this. Imagine what they'd write if they were on a message board, or their own blog. "Someone got killed at my school," "brought a gun to my school," "got shot at my school," "murder at my school," etc. Then I'd click through to their profiles, or contact info, and try every which way to e-mail them to ask if they'd consent to an interview. (Of course, once you have someone, fact-check it to make sure that the shooting really did take place, and if possible, get confirmation from a school administrator that this person was a student there. Teens do some weird stuff for attention, so ya never know!)
Also, check with your editor to see if she has any restrictions... the Dawson students are a good bet because there are so many talking about this online, but you'd have to know that your editor would be OK with the fact that (a) they're college students, not high school, and (b) they're in Montreal-- I assume the publication you're writing for is in the U.S. Sometimes that matters to editors, sometimes it doesn't.