Hello. If you are enrolled in a CBSE school then it will be quite difficult for you to change your subject now as you have been registered under maths as your subject. The examination of NEET requires you to have biology as your subject, it is mandatory to sit for NEET exam. But you can still talk to the administrative department of your school and seek their help. I will highly suggest you to talk to your school l, maybe they will be able to find any solution.