They must be mixing ability; it could be based on some sort of 'compromise' but it won't be good for students.Even then, I can't see how it would work. I was in the same class for all my subjects, but most of my friends had a mixture - some might be top set for maths but only third/fourth set for English, or vice versa. Unless you got rid of sets altogether and went for mixed-ability (which really isn't a good idea imo).
I'm aware of schools that do mixed ability for many subjects in KS3, but never for Maths, English or Science.
I've even known Computing and Sports to be lumped together (and yes that does mean all boy or all girl groups in Computing lessons)