It seems to be the story of a group of young kids who explore their school after midnight and make an adventure after meeting a human body model who comes alive at night. That body model is straight-up nightmare fuel. This can't be a kids' film, right?
The animator is Hitoshi Takekiyo and the studio is Koo-Ki. Never heard of that studio, but their style remind me of Studio 4°C and also the online series Charady no Joke na Manichi (but in a good way, Charady was terribly unfunny).
You can watch a second trailer for After School Midnighters after the jump.
For more information, see the ANN article on the second trailer here. Who else is interested in this kooky film?