Gakkou Gurashi: New Friends

Zombies reach Megurigaoka High School, when students were walking out the gate.
When more of the students and teachers that stayed after school get bit and turn, a group of unbitten students run to the back of the school and hid in the storage room that has it's own bathroom and several boxes of snacks and water.
How long can the ground live, and what if they find out they're people still alive inside the school.