After you go into that door, you see a whole new world, Mr. Long's World. 

Welcome:

Go into Mr. Long's School after your kid vanished while playing outside, Mr. Long's School has been abandon for years after everyone vanished without a trace. Go into Mr. Long's School to find your kid and find out what happened to all of the missing people.

Fever Dreams:

Have dreams of what your kid could have to do, But not all dreams are 100% true, so take the dream with a grain of salt, or you can think these dreams are 100% true. What ever you think, Mr. Long's School is still the best school Ever!

The End:

If you have read all of the stuff up there, then what are you waiting for, go play the game and have fun, Test what you have learned at school and see if you're smarter than a 1st grader.