Gakkougurashi!, Alice in Deadly School Recommendations
It's the same kind of story - zombies everywhere & schoolgirls trapped on the roof of their beloved school. Either wait for eventual attack or die from starvation. So they have to escape and live in this dangerous world. Apart from the identical theme we have the whole TV series instead of the short OVA. I recommend it wholeheartedly if you like that kind of reality - harshness of everyday survival and exploration of post apocalyptic world.
Both had girls trapped in a school hiding from the zombies out side, and waiting for help. Both are the zombies were originally people and the normal people can get infected. Both have that hopless feeling that help will never come and soon they will run out of food and water. I can't be certain if you like one or the other because it's seems like Alice in Deadly School is a bit of a rip-off Gakkougurashi, and some people might not like that because they might not the idea how there a little TOO alike but if you like one try out the other.