Hrm let me go back to my high school days. Ok, I'm there.
First let me say that in a situation where you can not leave, you die. Simple as that. Now we will assume that once those holding the lockdown in order have kicked the bucket that we can exit.
I think first you need to become a leader and aim people where they need to go and get them to gather supplies. I tell everyone to take the supplies to the top floor as if were not getting out, we should get up as high as we can and always have the entrance to the roof accessible. If the stairs can be taken apart, I do that and were pretty much safe. If not, I take supplies and work on barricading the steps (as I assume if were still alive the doors and windows are barricaded as thousands of zombies would eventually come crashing through). High schools normally have gym and shop classes so that's where we will find our weapons.
(After the military presence is gone. 25% infection rate infects them as well, and airborne infects the world. That military dies eventually. Might be a few hours... but they do die.)
Once we are relatively safe we work on an exit strategy. We get on the roof and look at what we are dealing with. If there are not too many we arm up and try to make a run, if it is heavily surrounded I work out a new plan. The first thing I can think of is live bait. Meaning getting items and hanging someone off the side of the roof to gather them all in one place. I think I am the one to be dangled (of course with a plan) and once the zombies are there, people run out of the door furthest from where the zombies are gathered. As they leave I climb the rope and get back onto the roof and examine the situation. If I have time to exit I do, if I see my classmates outside getting eaten I just sit tight inside knowing that all of the supplies are up top with me, the stairs are destroyed, the doors and windows are barricaded, and I didn't have to lift a finger to build a safe place to lay low for awhile.
