What Was the Name of That Book? Part 2

Some days I feel like this one is on the tip of my tongue; some days I feel like I'll never remember what it's called.

This is another book I read as a child. I know that I read it at my friend's cottage, so I want to say that it was between 1991 and 1997 when I read it, but it could easily have been published before that. It was a paperback, possibly pocketbook sized. There was likely a character, maybe a main character, who was overweight. I think she wore a school uniform. The most memorable thing about the book was that they called the bathroom a lavatory.

Does anyone remember this but me?