+0
.Students may not use the staff car park.

.Visitors may not feed the animals.

We can use may not in those occasions, but how about we only use "may"? Like:

.Students may use the staff car park.

.Visitors may feed the animals.

Are these two OK?

Thank you very much for your reply!
+0
These are correct and common and natural.

Students may wish to use the restroom before boarding the bus. (Ouch! this is a different sense of "may." Guess I'm having a bad day.)

Students may refer to their textbooks during the exam.
+0
In these sentences, may and may not are essentially opposites. Whenever you have this sense of may (have permission to), may not (do not have permission to) is also possible .
You may park here. = You have permission to park here.
You may not park here. = You do not have permission to park here.
may grants permission; may not denies it.
CJ