Is the student's enrollment in the university and his degree considered void, and his work an illicit gain, if his high school grades were based on a system that included buying grades, leaking exam questions, and cheating among students, in addition to benefiting from prohibited circulated compilations of aptitude test questions?

It is necessary to repent to Allah Almighty for the cheating that occurred in high school. There is no harm in continuing university studies with the certificate after repentance, as Ibn Uthaymeen fatwaed. Likewise, what he earns from his work after graduating from university is permissible, as long as he performs the work as he should. He must repent for the previous cheating, as Ibn Baz fatwaed.