If someone teaches your character and let's assume they have a lower skill in that
area. (The actual teach command)
Will that lower your character's skills?
Nope. You just don't learn anything.
Same if they are not marginally better then you. They can't teach you something if you basically are just as proficient as them. This is only code-wise though. You should not forgo or avoid 'lessons' just because you know that you won't be giving them a 'teach'.