(I think Teacher x Student can continue as a separate tag though)
If we get Teacher, it'd probably be better to ditch Teacher x Student. Combined with School girl, School life or College, Teacher alone should get the job done more often than not.
But what if the story involves only Teacher x Teacher and Student x Student yuri, but absolutely zero Teacher x Student yuri?
I agree with this. Logically, Teacher doesn't equate Teacher x Student. Even Teacher tag and a Student or School girl tag being present in the same story shouldn't imply there is any teacher x student content - especially since people are used to it being designated by Teacher x Student.
To me, the crux of the issue is that Teacher x Student informs of a central relationship in the story, whereas Teacher should inform of only of a non-minor character who's a teacher.
If the problem is too many tags in the system, adding Teacher won't help. Removing the more useful Teacher x Student to make room is counterproductive.
If the problem is too many tags per story, you could make it a policy to limit the use of Teacher to situations where Teacher x Student is not applicable.
Student x Student = School life, College. And sure, it's possible that my suggestion wouldn't work for 100% of the stories (I even specifically mentioned that it should work more often than not) , but it should work for most of them. Just having School life should work for most of the cases in which there's a non-minor teacher character who isn't involved romantically with anyone in the story (as opposed to how I imagine the tag used: a teacher who's part of a couple in the story, whoever their pair may be).