Should I pay the full tuition fee for the Quran memorization classes, given that I only attended twice a month and donated the amount to another mosque, and I still feel guilty?

The aforementioned subscription is a binding lease, and the full rent is incumbent upon you. Give the money as charity on behalf of its owners, and if you find them, give them the choice between affirming the charity or paying them their due. Giving charity does not absolve one's liability except when there is despair of reaching them.