These trespassers can include a school's own suspended or expelled students, students from rival schools, irate parents seeking revenge against a student or employee, gang members, or even drug dealers. It is logical, then, that if a school were able to carefully control exactly who was able to enter the campus or school buildings, security incidents would drop significantly. This is easier said than done.
Schools can often prevent or discourage the casual intruder.
Signs that inform visitors that all vehicles brought onto campus are subject to search by the school. A guard who is checking identifications at the main entrance gate to the campus. Vehicle parking stickers so that any vehicle found parked on campus without a sticker, other than in the monitored visitor lot, is subject to being ticketed and towed. Uniforms for students, which make outsiders very identifiable. Minimal numbers of entrances to the campus and to the school.