If i ever turned up late to lectures we are told to leave even if its only 5 minutes late, he is paid and employed to educate you thouroughly. If this course was full time then 20minutes late is neither here nor there, but by the fact you do 6 hours per week, your study time is extremely important.

If you get any shit from him, just be nice then inform headteacher, principal of the situation. Don't worry about it bruv!