This is just really not ok, IMO.
Several years ago, a teacher was using my son as a tutor was baffled when I was angry about it (doubly angry, actually, because they had promised me that he could work on out-of-grade-level math when he had finished his own stuff, and she had him tutoring instead). She told me that she'd "always been taught to use my best students," and that this policy was common among teachers.
She honestly didn't see anything wrong with what she was doing. I suspect that, from her perspective, he knew the material and therefore simply didn't need more instruction, and so asking him to help other students was a good use of his time.
At that point, she simply didn't see that he might need more, especially because he was already doing reading and LA a year ahead.
