If you think about it Pennywise chose the hardest group of kids to terrorize- pre-teen boys, who actually enjoy being scared, think blood and gore are cool, and whose rising hormone levels leave them eternally horny.
That's not to say pre-teen boys aren't easily haunted, but the hauntings would have to pertain to being embarrassed in front of a girl at school, or getting an "over eager visitor" during class, and Pennywise does sewers, not schools. (comic by JHALL)
-Via Dorkly