He thought that he could/should at 15 drop out of school, he was excited about the idea of doing so, since he thought he doesn't need school, that by being an Avenger he could make a living, maybe even a successful living. The moment was portrayed as being a mistake on his part and by the end of the film he decides not to join the Avengers, he's got more to learn on his own and maybe he shouldn't rely on membership (although that part was less clear). But was it either believable or good for the character for him to have had that moment, even that motivation (he badly wanted to join throughout, although only in that moment did it explicitly seem so much about his personal wellbeing)? Was it just a slightly-more noble way of exploiting his powers than what he (presumably) wanted to do earlier, just be a celebrity (and that badly backfired)? He was also highly tempted to and came close to following Ned's advice, using his powers and persona to get Liz impressed & interested in Peter and romantically connect with her.