Another factor is probably panic. The guys may have had their rest, but remember they're not trained soldiers for a crisis like this one, they're just students and a teacher, they often make decisions which can endanger them. Like Takashi having suicidal tendencies because of Rei there injured and can't get up(probably she broke her back or knees), where instead go to through or over the fence, but as we see zombies are everywhere in just a few seconds.
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Vampire+Sisters
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