There are several issues to consider:
First, the legal nature of the grant. The grant may be on a contractual basis, i.e. there is a civil agreement between the student and some state entity. The sovereign does not impose his power on his citizen, but steps down to his level.
However, the grant may also on the other hand on an administrative basis (an acta iure imperii). The citizen applies to his sovereign to provide him with payments to cover tuition in the foreign state. The sovereign then uses his powers to impose on the applicant his terms.
The difference can become relevant where the citizens does not comply with the terms of the contract or the act respectively. In case of a contract, the sovereign has to bring a claim in civil courts to obtain an enforceable judgment against the student. If the basis of the grant is an administrative one, however, the sovereign may not need to bring a claim in a court of law. He may instead have the power to issue another administrative act, which demands the payment, and, most importantly, can be itself enforceable against the student, if he does not contest it. Where he does not, a court of law is not involved, a judgment is not given. The state merely sends out a bailiff and enforces the repayment act.
This distinction becomes even more important, where the student is domiciled in a foreign country. As DaleM pointed out, the judgment based on the contractual obligation may be enforceable in foreign countries, as it is of civil nature and in money. By contrast, a judgment for specific performance (i.e. performing an act other than payment) is usually not enforceable. Where the legal nature of the obligation is an administrative one, enforceability in a foreign state is usually not possible. A counterexample would be the Directive 2010/24/10 concerning mutual assistance for the recovery of claims relating to taxes, duties and other measures in the EU.
It should be pointed out that the sovereign that granted the grant and wants to claim payment under the "penalty clause"* may also bring a claim in the courts of the state where the student is domiciled, resident or merely present. For these courts can have jurisdiction over the student. Whether this is possible may again depend on the legal nature of the grant.
As far as you state that are co-signors and they have passed away, their estate may be liable for payment under the "penalty clause"*. If they have left the country, the same arguments apply as in regard to the student.