I think you did the right thing calling the police, I would ask for a report to be written on the incident also from Boulder police and get a copy.
Each school is difffrent, my son is in a charter school and the front entrance you enter a little room and must present ID and have a valid reason to get the vistor pass and then you are beeped into the school.
I am sure someone could sneek in another entrance, heck people break out of prisons. So almost no place is absolutly secure.
If they had no pass and the school did a sweep, they should have called police.