When you start at a tag T and type in S as a synonym suggestion, then you propose that T stays an actual tag, and S becomes merely a synonym of T. This is typically noted T <-- S.
This means everything tagged S will/would be (automatically) retagged to T. Being a synnonym is highly asymmetric here, and thus it is also not possible that one has both directions at the same time.
As you suspect, in the case you mention it was created the wrong way around. If the current one would be approved we would get college <-- undergraduate-education and thus keep college, which is opposite to the discussion.
I do not know how to get rid of it other than by dwon-voting; it appears one cannot cancel ones own proposals.
As a very general note: the synonm creation only via proposiing and voting on main does often not work well as there is not enough visibility; it can be more efficient to propse synonyms on meta, and we might want to create a dedicated thread for this.