High school students can look imposing to a nervous cop, yet the armed principal knows the kid in the black hoodie is just Norman the nitwit junior. I attended such a high school & the principal knew the name of every kid in school, without doubt.
School staff will be far more likely to recognize a stranger, too.
I’ve always thought that janitors and maintenance men would be ideal security men. Having done the job myself I can say that few people know the school building better and they aren’t tied to a classroom.