Surely you're aware there is NO CORRECT ORAL GRAMMAR taught in the UK. Simply some uses are more standardised than others.

Received pronunciation is a learned pronunciation, usually not naturally occurring outside the upper classes (who are undesirable because they often perceive people like things they can own and aren't grounded in reality). 'Correct' verbal English, as taught in the US and Australia, resembles the English spoken in RP. Therefore 'correct' verbal English stinks of the upper classes, who few aspire toward.

This is why 'May I...?' was laugh-out-loud funny when I asked 'May I have a carrier bag?' at the supermarket. I'm not upper class and it seems pretentious to use May I...? In the UK, the practice is that 'may I' is posher than 'can I,' not more polite.

While this is on a tangent, the teacher was probably unaware that stay sat was somehow not correct (and remember the British don't have a correct verbal English). The British are not taught grammar rules and many people, teachers included, have no idea what the continuous form is and isn't and when it ought to be deployed in speech or in writing.