I've just started year 13 and I'm doing my maths the way your school does it. I'm doing maths, further maths, physics, eng lit. it's really not that hard, I say go for it. If you're looking at top unis for maths heavy degrees it will be very very advantageous. doing fm now will make your life in uni a lot easier. also doing 4 a levels is not that daunting, besides when you're doing maths and fm in this way it feels like you're only ever juggling 3 subjects at any one time.