Not sure why they'd cap MX2 at that range, if there was a greater demand for MX2 classes then they should be able to supplement them with an extra class or so. Also there's going to be people dropping MX2 naturally whether it be at the start of the course (the bulk of people) or to the middle even if they're hard working so it doesn't make sense. Personally I would still go to the head teacher and inquire if there's any way, it doesn't hurt to try trust me, I think you'll find some teachers have a soft spot for those who have worked hard + your apparently rank 10 for MX1 so you would fit the criteria?
It does suck because if you are to a 4U capability you will not be rewarded, and you don't have the same incentive as your 4U counterparts for grinding maths.
If you tried and the teacher won't budge, then you just have to take the situation in and realise its not the end of the world. Think of the benefits for not taking 4U, you can focus on your subjects such as chem/phys etc that demand more focus and in that way you can get more done there.