My girl was in pre-nursery last year and she was in the international class so supposedly the teachers should only speak English / Putonghua (PTH) to the kids though of course, you expect they have to use Cantonese now and then.
She had 2 PTH + 1 Eng teachers last year if I remember correctly. Starting K1, they have 1 Eng + 1 PTH teacher if you are in the International class.