It depends on how far you want to advance their stories and what you consider canon / NTR. Most of the girls are borderline free use at some point in their story. I didn't do extensive testing, but the guide often refers to seeing certain NTR events in order to advance the plot. Like Rain was saying, so long as you don't witness events technically the girls corruption never increases so you could logically argue that it doesn't happen. But in a game where the schedules are set in stone and you know a certain area is off limits on certain days, is it really the same?
To directly address your question about the nurse and the teacher; it depends on which teacher.
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So until further changes/additions are made to each character's story line, there you go. There's only 2 girls at the moment who will "commit" to you via dialogue once their love is high enough and it's your childhood friend and the masseuse.