Someone who is paranoid has an irrational and obsessive distrust of others, like your paranoid friend who worries that everyone from the principal to the neighborhood pre-schoolers are out to get him.
A paranoid person suffers from paranoia, the unreasonable fear or irrational belief that other people are plotting to harm him or her. How can you spot paranoid people? They tend to obsess over other people talking about them. They believe there are no accidents or coincidences — the cafeteria running out of fries just as they reach the front of the line — just plans to hurt, embarrass, or otherwise bring them down.