while it may cost some money, I'm sure you can find somewhere else to sit your exam in June (or even next January) your teacher can only prevent you from taking the course at your school/college not sitting the exam. Not living in england I'm not certain you can do this there but you can here, I would assume you can find somewhere else which lets you sit the A-Level exam. It's best to call your A-Level examination board and find out if there are any places where you can sit the exams without being affiliated to a school/college. It's worth a try if you feel you can achieve a grade in maths.