The grammar says ''may/might do'' is used in talking about possible actions or happening in the future, while ''may/might be doing'' is applied in describing possible plans. They can be both fine in many cases with little difference of meaning.
But in the exercise:
What's Julia going to do when she leaves school?(go to university)
She hasn't decided yet. ____Answer_____
My answer is "She might/may be going to university", but the standard answer is "She might/may go to university'', so I wanna know if my answer works, since it's talking about future plan, and according to its own introduction, it should have worked. And I wanna know the slight difference between these two expressions.
I would not use may/might be +ing for possible plans. I would use it like this:
a) What is he doing now? I don't know. He might be sleeping. (possible present continuous action)
b) What might you be doing tomorrow morning? I haven't decided yet. I might be working in the garden. (possible future continuous action)
In your case, I would say "might go" because neither a) nor b) apply.
Contrast these with definite situations.
"I will go to university" / "I am going to go to university" - definite decision/plan
"I will be going to university next weekend" - definite future continuous action