Girl didn't fell in love with the teacher, she fell in love with sex, proven but her staring at man genitals even if it wasn't the teacher. Her parents managed everything she did, so she was never totally free, bottled up feelings and stress, secret sex with a adult for money made her able to release it, even likely she was a born sex beast. I think is unlikely she will go with someone else unless the teacher have to move far away or for some reason cuts her off, that he have no reason to, I bet he could pay her just 500 yen or with Plan B and she would be thankful. Or, as I said she doesn't love him, she develops feelings for someone.