Does the letter provide any further detail or does it essentially say ineligible claims were paid and you owe them the money with no other info? It sounds to me like either they messed up and should have terminated your coverage earlier and didn't or they kept your coverage active based on inaccurate information being provided to them and have somehow caught it later.

Assuming the plan is through one of your parent's employer, generally there would be some kind of annual process where they would have to confirm that you are still enrolled in school (and therefore still eligible for benefits per the contract).

Depending on the circumstances there may be an opportunity for your parent to ask their employer (the plan sponsor) to review the situation and ask the insurer to allow the claims to remain paid on an exception basis; however, if false information was provided through this enrolment process (i.e. they said you were still in school when you were not) then you would likely be better off to just repay the money rather than bringing further attention to the issue.