At the very end of Jack's mission at Grissom Academy, you choose whether to send the students on the front lines, or use them as support roles. Sending them to the front line has them killed at the end, but they live if they're support roles.
If anyone knows the plot flag or id to do this, it'd be really helpful
Best Answer
http://pastebin.com/raw.php?i=zv8mxcxs
This is a list of all plot flags/IDs as found at http://forum.bioware.com/topic/306020-mass-effect-3-modding-thread-last-update-14-05-2012/. I'm having trouble deciphering which one points to Jack's mission though. I'm guessing it's OmgJck, but I have little to back that up other than "Jck" in the name.