Different schools get different levels of funding, and they may decide that there are more important things to spend money on. There might not be a need for real security in some schools - my high school used to have no security. You walk up the driveway and you're on school campus and you can get in anywhere - no securit guards and no barriers. You's only have a problem if you were not in uniform (unless it's a mufti day in which case literally anyone could come in) or found to not have a visitors pass in which case you get escorted to reception. But a high school 10 minutes down the road had ID passes, airport style detectors and beefy security guards. This school was the worst in the area for many years, and students carried weapons - it wasn't nice. Whereas my school was full of "respectable grammar school girls" and ranked joint top 8th State School in the country last year. It depends on the reputation of the school, and the status of students there. Like if a royal went to one of those schools it'd have a hell of a lot more security. There're a lot of factors involved in this :/
Some schools have more problems than others. I attend to a school where there is a lot of drug issues and all that so they have police and people always watching around the school. But there is a another high school like 2 blocks away from it and it's like, nowone watching it and it is considered a good school. So I think that It's just depending on the school and it's issues.
Maybe the school doesn't have as much as the other even though they are in the same area , or maybe the person who has to buy stuff for the school is a penny pincher lol.
Depends what happened at the school or not