Is financing tuition fees by installment payment over 12 months without increase, with a one-time administrative fee of 15% being charged, permissible (halal) or does it fall under usury (riba)? And is the percentage that the app-owning company takes from financing entities (banks and financing companies) in exchange for marketing customers to these entities permissible?

It is permissible to act as an intermediary between students' guardians and financing companies for a commission, provided that the loans are permissible. This means that the percentage taken (administrative fees) must be actual charges for the loan service and not a cover for usurious interest. Any increase beyond the actual services is forbidden and considered usury (riba).