I never understood the harem tag sometimes. A harem is when a person is dating or is married to multiple people(with their acknowledgement and consent, if not then it is cheating). Multiple people liking someone isn't a harem. It's just multiple people liking someone. None of this matters...
I wonder if Sensei is secretly recording her so she can show it to Rei to isolate him more. They need to get Gen in on this. He would definitely go after the teacher.