Jason came to Grissom when he was six, a shy youngster who barely spoke a word. His fellows observed him always reading and studying battles, with some even calling him a brownnoser. He proved competent enough to take on leadership roles, however, when the occasion warrants it.
During the 2186 Reaper invasion of the galaxy, Cerberus attacks Grissom Academy to forcibly recruit the gifted students there into their troops. If Jack is alive, Prangley is among the students she's protecting, and he was about to be obliterated by incoming Atlas fire when his teacher steps in to intervene. Jason later aids in the firefight when Commander Shepard arrives and gives him and his teacher the opportunity to retreat to higher ground, shooting biotic support strikes against enemies in his line of sight. After the battle, Jack derides Jason's biotic fields as "weak", berating him that Cerberus won't lie down out of pity like a girl he once took to prom. He considerably shapes up his performance afterward in the succeeding battles.
If Jack did not survive the Suicide Mission, however, Prangley interacts with Shepard in her stead when they arrive to evacuate the students. The Commander finds him stepping up to save Ensign Rodriguez from incoming Atlas fire. Jason informs Shepard that their commanding officer has been killed, which left the young man assuming he's in charge. Shepard tries getting the recruits to immediately move out, but Jason speaks on their behalf that the students need time to recharge after burning a lot of calories; biotics being a strenuous activity.
If Jack died, Prangley later dies trying to protect Ensign Rodriguez again from Cerberus troops mere meters away from the escape shuttles they hijacked. The survivors eulogize him aboard the shuttles afterwards. Shepard uses Jason's memory to incite his fellow students to fight, either by using their anger at losing him to take on a more offensive role, or the fact that his act of saving lives should inspire them to follow his example.
Several datapads left by enemy troops around the Academy indicate that Prangley is especially targeted as a prized acquisition for Cerberus.