I would disagree under certain circumstances. "Class" as in 'one class in a day' is certainly countable, but class as in 'a course in general' is not countable. It would be correct, regardless of region, to say, "Esther is teaching Mathematics".
That would be like saying, "Welcome to class' is incorrect, you have to say 'Welcome to the/à class". When it is not incorrect, in both cases class can (under certain circumstances) be used as an uncountable noun.
Because there is no way to differentiate, both are correct, and therfore, should be accepted.