Any of these might be appropriate, depending on local standards. Nobody (or very few people) in the UK would say 'restroom'; and, in Br English, (in the south of England, at least!) 'bathroom' refers to a room in a private house - a public place doesn't have 'bathrooms'. But that doesn't mean it'd be wrong in HK. Ask other teachers at your school.
In my youth. 'May I be excused' was the norm - in fact, it was so clearly a euphemism for 'toilet' that an adult might say to a child (far from any such facilities) 'There's nobody around, you can be excused behind that bush'.
Student or Learner