Rowena and Gertrude
There have been plenty of posts about Rowena and whether or not she should have come back at the end, like Gertrude did.
My two-bits:
(Okay, declaration of interest: I'm a high school teacher. I see it from that perspective.)
Rowena was a very gifted girl who had the dreams and the talent to go a long way (even though few people ever make it big). I don't see Holland as being anything more than an accompanist to her talent. He saw the talent, she knew she had it and she offered him a final chance to follow some "dream". In her youthful naivety, she did not see what was obvious to him: he had to stay at the school and with his family.
Gertrude, on the other hand, was your typical insecure student who needed her teacher's influence to break free and "play the sunset" (which is a bit lame, but the point of the dialogue is clear enough). It is entirely fitting that she should return as Governor of the state, owing her first experience of self-confidence to the efforts of Holland. She was the kind of student he did the best work with, and they filled the hall at the final concert.
In short, Rowena was one of those flashes of brilliance that you see from time to time.
Gertrude was the student who first made Holland realise what he could really do, and it had nothing to do with fame or brilliance.