Why do my classmates like using my Chinese name instead of my English name?

Two contrary reasons. In the particular context you’re detailing (them laughing), one is likelier; but both should be stated for others coming across this question.

One tendency is mockery of the exotic; teenagers in particular have a strong, even brutally, conformist ethic, and they deride names that they find out of the ordinary.

The contrary tendency, which you’ll find more with adults, is what its proponents will call intercultural respect and its opponents political correctness. It is a rejection of assimilation, by people aware that assimilation is a cultural imposition on minorities. This can lead to overcorrection (people expecting chopsticks in Thai restaurants), and it can lead to paternalism (they may insist on using a Chinese name for you, thinking they are respecting you, even if you’ve told them you’d much prefer them to use an Anglicised name). There’s some exoticisation going on there too.

That tendency, for better or worse, seems to be more an Anglosphere thing than elsewhere; check the controversy over what to call the capital of China, for another example.