Depending on the student's age, the teacher (as well as any other adult in their circles) may be doing tons of other things that the student is prohibited from. I'm not sure how this one is specifially different and problematic.

Dear student, please do not get your teacher, who is apparently a good teacher, fired or worse for something that you never even noticed before. Whatever your teachers do outside of work hours, and doesn't harm you or others, is none of your business.