The term 'major' is used in N. America, but not in Britain, at least not while I lived there.
A 'major' is basically a main field of study. It's not the same as a course or a specific subject. You will typically take several courses on various subjects that relate to your major.
So it would be better if I said: Our high school offers different courses. AND I chose math because there were no other interesting courses to choose from.
A high school in the US will require you to take a variety of courses. For example, it's mandatory to have a certain number of classes in math, English, writing, history, a foreign language, and physical education. The actual requirements will vary state by state and school district by school district. Where I grew up, a foreign language was not mandatory, but physical education was required ever