What does it mean when teacher say good for you mean?

Answer 1

It means the teacher was pleased and is implying you should be, too.

Answer 2

Confirmation of having made the correct choice. A form of encouragement of sorts, to keep being like that.  Unless said sarcastically, then it would mean precisely the opposite: that what you did was not good and not worthy of repeating.

Answer 3

what is another phrase that starts with GFY

Answer 4

It means 'well done'.