I was talking to someone just yesterday who was saying her school didn't allow you to do a mix of arts and science A levels - it had to be either/or. She wanted to do Maths, FM and Music but wasn't allowed to do music.
Is that recent, or in the past? 40 odd years ago, I don't remember much mixing in my 6th form (though I don't know if it was barred) - and 4 subjects were only allowed if they were double maths, physics and chemistry. Nowadays, DDs school definitely allowed mixes - eg one of DDs friends did double maths, physics, French and eng. lit (possibly dropped the latter) - just as well as she decided she wasn't so keen on her maths degree and swapped to MML.
I suppose in very small sixth forms timetabling could make mixing problematic.