Nope.doors stay locked...interior and exterior at all times. Public entrances especially. No exceptions. escorted at entry point if some parent or delivery needs entry. And escorted back out.
Armed select staff. Preferably with long guns (secured) they can get to. This is perhaps the most difficult part as it requires dedication, training, and willing to assume the liability. Not to mention the problems with staffing shortages, calling in sick, substitutes, field trips, etc, etc.
the above are doable at minimum expense. There will still be lapses and failures as the best laid plans of mice and men will certainly come into play.