As a teacher, even if your student tries to inappropriately come on to you, you are supposed to shut that down. Even if she has been a good influence on him and helped him pursue a talent, and even if she hasn't outright pursued a relationship with him, her behaviour in the beginning where she would tease him suggestively, sometimes physically, and encourage boundary-crossing is what makes this dynamic feel like grooming. It's not suddenly sweet just because she accidentally caught feelings when he starting giving as good as he got.