The likely reason they are capitalized is that they are probably official course titles. For example, his official report card probably lists grades for "Maths," "English," "Science," and "Art." To be fair, the capitalization rules for English are complicated. A printed book like this one was probably "copy-edited," who is an expert in capitalization rules and has a "stylebook" to consult for difficult cases. But this certainly "looks right" to me. The capitalization could depends on the context. For example, if someone wrote to me "Did you study science in high school," they would not capitalize the word. And I might reply, "Yes, I studied science. I took General Science in 9th grade, Biology in 10th grade, Chemistry in 11th grade and Physics in 12th grade." That's because there was not any course named "Science." But there was one named "General Science," so I capitalized that. And I capitalized the specific names of the specific courses I took.

Here, it is essentially implied that these are the names of the courses. As such, they have each been capitalized as proper nouns. It would have been just as valid to treat them as common nouns, however. In that case, they wouldn't be capitalized.

All proper nouns are capitalized, along with the first word of each sentence.