He got stabbed by a Lakehurst student, after he went looking for Liberty. Emma + Manny threw a 'birthday' party for Liberty at Emma's house. It was JTs idea. First he talked to Liberty, who got mad at something he said and left the party. He was debating on weather he should be with Mia or Liberty, and after talking to Toby, he figured out that it was Liberty. He went looking for Liberty, and found two Lakehurst students by his car. JT was about to open his car door, when one of them pulled out a random knife and stabbed JT in the back, for no reason what so ever. That's how JT died[: Hope it was helpful!
Degrassi: The Next Generation season 6, episode 11 [Rock This Town]