Is money earned from a job after studying at an institution where admission was granted based on an unofficial absence considered unlawful gain?

There is no harm in continuing your studies, as there was no deception or fraud involved, and because students might be authorized to not record absences. Furthermore, laws are binding if they are enacted for benefit, not for creating obstacles. The money earned from work is permissible if the work is performed as required, even if there is some negligence in studies. It is advised to disregard whispers (waswas) and strive diligently in your studies for the benefit of yourself and the Muslims.