Under what circumstances are you referring to - that you would need to ask to enter a classroom?
In the US, you would just enter and say nothing. This, of course, is when you are supposed to be there. :)
If you are not supposed to be there, you would not enter at all. In this case, if you asked, you would be refused.
Is there a specific circumstance that you are referring to? (Maybe following the rules of another culture/country, but having to speak in English?)