"Honestly, I suggest the Glock over the Magnum. The magnum does more damage, but the Glock has less kick, a large clip, and the gun is much smaller and lighter. Holding out now, I would say it depends on how big of a haul of food you brought and how long said food will last. If all the food is non-perishable, with proper rationing, we could last weeks to years depending on how much food we have. Water may be an issue, but if the haul was big and we don't all mind smelling like a dumpster, we could hold it out about as long as the food with rationing.
Honestly we should be ready to pack up and flee at any moment. If we keep the U-hauls and pack everyone in like sardines, we could take the small children, most of the staff, and the more useful of the students. Everyone else though." Jack sighed "Would either have to find vehicles of their own and follow us, or split up and form their own survival group. No matter what though, that transmitter stays with us." Jack moved to sit down.
"The only way we can hold out indefinitely is to secure all of the school ground, and use the roof and windowsills to catch water when it comes, and the grounds to farm. Rationing would be a fact of life. We would need some kind of stone or cement wall to keep the zombies out, and heavy doors that we can easily lock as well. The school would need to become its own citadel, and we just don't have the resources to do that. I give us maybe a month before we need to flee as fast as we can. Assuming the military has leveled the city or saved us by then."