Is there any objection to a teacher giving lessons to students for a fee calculated by the hour per student, or to educational centers hiring teachers to provide courses to students with the teacher's fee determined based on the number of attending students and an imprecisely defined class duration?

Both forms of transaction are valid leases, as the rent and the benefit are both known. The benefit is determined either by its inherent nature, by naming the specific work, or by specifying the duration. The condition of being known, which prevents disputes, is met in both cases. Therefore, it is a condition for a lease that the benefit must be known in a way that prevents disputes.