You don’t need a mufti to tell your that these students are cheating. They are committing a grave sin, and you are committing a double sin by encouraging them to cheat and making money out of it. The income you earn through this fraudulent work is pure filth.
The Prophet (peace be upon him) warned us against unlawful earnings: He said Allah does not hear prayers of such people. He teaches us this principle by citing the case of an individual who undertakes journey to perform Hajj and raises his hands in the sacred sites to Allah, calling out saying: My Lord! My Lord! "How would Allah answer his prayers when his food is earned through illegal ways, his clothes are from unlawful earnings, and such income also nurtures his entire body?" (Reported by Muslim on the authority of Abu Hurayrah).