Depends on the subject. e.g. GCSE Maths in Edexel is quite easy but the same linear AQA one's papers are a bit harder.
I'd say most schools choose AQA is because they probably find their specs the most interesting to teach to a bunch of bored teens.
At A-Level, tutors know that the students are actually going to be interested, hence the use of more exam boards, I know that for my A Levels starting next year I'm sitting Edexel, AQA and OCR as opposed to my gcse's now which most of them are under AQA