What is the Sharia ruling and expiation for the sin of using students' funds for personal gain, bearing in mind that I did not take payment for my efforts and that the group has disbanded?

The agent is a trustee, and it is not permissible for him to act without the permission of his principal. Your disposing of the association's money without the permission of its owners is a betrayal and an aggression. What is incumbent upon you is to repent to Allah, return the money you took, or seek absolution for it by giving it as charity to the poor or for the benefit of Muslims.