First of all, they don't. Teenagers are regular people. They do not believe everything their friends say. They argue about facts, gossip, games, etc. They do seem to have less resistance to peer pressure than most adults. If everyone in the room is reading and studying, the most likely thing the teenager will do is read or study. Likewise, the same can be true of some risky or unsafe behaviors, such as sexual practices, drinking, taking drugs, fighting, etc. Competition is probably just as likely as peer acquiescence especially among teen males.
It is important to remember that teenagers have common concerns and they seem to rally around these topics.