"The School" was originally published in 1974 in The New Yorker, where it is available to subscribers. You can also get a free copy of the story at National Public Radio (NPR).
Barthelme's story is short—only about 1,200 words—and really funny and darkly funny, so it's worth reading on your own.
The story achieves much of its humor through escalation. It begins with an ordinary situation everyone can recognize – a failed classroom gardening project. But then it piles on so many other recognizable classroom failures that the sheer accumulation becomes preposterous.