It was a single classroom but, with the exception of third year students, it had everyone from year one to year six.
??? but right behind it were beautiful grassy mountains with chestnut trees and ...
I'm struggling with the 運動場もテニスコートのくらいでした part. My guess is that it's saying that the classroom also acted as a tennis court and sports ground, but I can't convince myself 100%.
Also I don't think I've ever seen くらい with の before. Can I omit の without consequence?
Edit: The part I missed out is
...and in the corner of the sports ground there was a cave where cool water bubbled out.
So I suppose this makes my guess completely wrong. Now I'm very confused.