The teacher isn't MC's girlfriend or even a love interest at this point. So I don't see why it's NTR just because it's not with MC. It is however nasty and I have less trust in the author if they are into that.
@Giga
Never lol. This is how this harem-y formula works. Everyone gets to have a go at doing something lewd with MC and everyone is supposed to pretend it's never his fault because he just sits there like a vegetable and takes it.
This is a beautifully made stylish yuri version of Hachigatsu Kokonoka: Boku wa Kimi ni Kuwareru. They have the same premise. The art is great and the flow of panels is beautiful.
@androssia
While I agree that people freak out too quickly. The premise is that one is watching/recording and the other doing. Which lends itself to the idea of recording her with someone.