students usually like free stuff but app requires to make money so what would be the best revenue model that suitable for students.
I think you've got a great answer by Assaf Ben-David.
In addition, it might be worth considering partnerships and who else might benefit.
For example, could your client also/instead be the educational institutions where the students study?
Is there a benefit to them somehow?
It's true that students are cost-sensitive but can you offer them something they can't live without? Worth considering and refining the value proposition and the price point.