Translation:That teacher's class is not fun.
If I'm not mistaken, this sentence could also refer to multiple classes, in a more general sense. "That teacher's classes aren't fun." I.e. don't take from that teacher. I was pinged for that answer.
I also think it should as じゅぎょう means lesson and class. Maybe there are certain conditions where you cannot use one or the other. I don't know from here, sorry, but would like it if someone briefly explained. Thanks.
There is no indication as to the plural of class or lesson and the singular. So "That teacher's classes aren't interesting" is just as valid as "That teacher's class isn't interesting."
"That teacher's classes are not fun." is accepted, so if "That teacher's class isn't interesting." is accepted, "That teacher's classes aren't interesting" should be to. Report it.
That would be "that class's teacher."
With の, whatever precedes it (in this case, 先生) possesses whatever follows it (じゅぎょう).