"The teacher waits for a student" is also correct?
I'd say yes, but it prompted that it was wrong :(
Because egy is used here, a valid (and possibly more accurate?) translation should be "The teacher is waiting for one student." Right?
It really doesn't sound as if that was the intention here. Egy diákot is unemphasized here compared to the verb, it's highly unlikely you would do this if you wanted to refer to the number of students.