book set in childrens home/boys school/boarding school in the uk (ithink)

[Ffion]

the book is set in one of them place i can't remember and they find some underground sort of tunnel thing or it was on the land, one of them nearly drown and a teacher was behind it. i don't remember much and it's a long shot but worth it. i think it was ya/children's/teen book. the cover was blue if that's any help. any help would be appreciated.
thanks.

[Ffion]

also it was a bit bigger in size than a normal paperback

[Lobstergirl, au gratin]

When did you read it? Series or standalone? Author female or male?