I don’t think you understand what goes into hiring a security guard. They could not just hire one guard, they would have to have a guard there for more than 8 hours a day (and overtime is too expensive so you need more than one) and depending on the size of the campus you would need more than one, depending on your area unarmed guards make 10 to $12/hr (armed guards get more) and some get benefits, and oh yeah, you have to hire your guards through a security agency, the security firm designs a security plan for your school, because people at the school do not have the expertise.
I also don’t think you understand what sort of legal liability that this school would incur if they had a credible threat, like this man and didn’t do something and there was an incident. It is called negligence. Calling the police the first time does not free you from negligence in the future.
Oh and just because people are sending their kids to private Catholic school does not mean they can afford the increase they would need to pay for a security guard, that is a terrible assumption especially since many kids at Catholic schools are there on partial scholarships, especially need based scholarships. Especially since there is a free option down the road.
I think the school had no good option. They could raise their prices and lose their students and have to close, (leaving no one with a job). They could not rehire this one teacher, a horrible choice to be sure, or they could do nothing and risk an incident, and possibly getting sued, and thus having to close. I am guessing they were given the options by their lawyer and they chose this one, which shows you how legally risky (or an impossible business feat) these other choices were.
I know of Catholic school teachers (music) who don’t know if they will get their contract re-offered, because enrollment is down so much, they don’t know if they can pay him. The Catholic school in my area, (well next to my area) is in a very nice rich suburban area, and they are closing next year, because of lack of enrollment. These are very real problems for Catholic schools, regardless of the area or how “rich” you assume people to be.