why are the coffins stacked up against the schoolhouse?

The coffins are foreshadowing as well as symbolic. They are there for most of the men who will not come back from battle alive. They are an ominous symbol of impending death for Paul and his comrades,

“’They’re for us,’ growls Detering. ‘Don’t talk rot,’ says Kat to him angrily. ‘You be thankful in you get so much as a coffin,’ grins Tjaden, ‘they’ll slip you a waterproof sheet for your old Aunt Sally of a carcass.’