I grew up in Singapore, where it's mostly English - actually, Singlish lol which is some extremely slang-y mishmash of English, Chinese, various Chinese dialects, Malay, and probably some Indian dialects thrown in there too idek. classes are in English, but we start learning Chinese / mother tongue right from primary school.
as for HK, i'm in an international school, so yeah English, but on the streets Cantonese is definitely used more - and I think local schools use Cantonese too. I don't speak Cantonese but I look Chinese, so it gets a little annoying sometimes :/ I'm going to college next year, so I think I'm in grade 12 now? called year 13 in my school (and the IB is eating my life D:)