Why would the school need to hire armed guards when the secret service is armed and at the school.
So...the school is already armed by the SS.
because when the shit hits the fan at that school the SS has a job to protect 2 girls and get them out of harms way. Something I am 100% sure they are able to do.
However, protecting the rest of the school is not their job and likely low on the list of priorities. My guess is that their first goal is to GTFO of the building with the girls safely.