Actually, the after hours classes are common in the 'better' schools throughout Thailand. Many teachers double their salaries with a few extra weekend or evening classes. The more unscrupulous teachers will let the students or their parents quietly know that test material will be covered in private classes but not in the regular classes. The next time you see a Thai teacher aparently wasting time in class, it may be because she doesn't want to do too good a job there and spoil her cash cow.