The courses were taken, the education was received. This would just transfer the cost of already provided services to the general taxpayer. It would make nobody more educated as a result.
If you are trying to argue that the increased tax levied on the particular indebted people with a better education adds up to the exact amount they owed, someone would have to prove that. Additionally, this additional tax from better education was never intended by Congress, when the tax rate was calculated, to go to paying anyone's previous education debt. So other services (arguably benefitting the entire population) would have to be preempted to pay for it. Either way, you'd have someone else paying for the debt.