This is all irrelevant. You’re talking about individual situations. I’m talking about a supposed general mandate at a private school which allegedly prevents students from signing early in December. Credit status is a side issue that has nothing to do with my point that I’m questioning whether Aquinas really has this policy (I’ve never seen where this is stated) and if they do, how they can legally enforce it.
The answer likely lies in your first post on the subject -- freedom of contract. Any policy STA (or any other private school) has about athletes graduating early or signing an LOI early would likely be covered by the enrollment/attendance contracts between the school and the parent/guardian and/or student. And even if that policy is not specifically addressed, there is likely some catch-all provision in the contract. So if, for instance, a school has an enrollment contract wherein the parent and student agree to abide by all formal policies and procedures of the school, and the school adopts a policy/procedure regarding early enrollment or early signing, the parent and/or student is arguably bound by the contract they signed not to enroll early or sign early.