Instead of a free trial, maybe offer the trial for $5, or as @Venonusa mentioned, a prerecorded video demo to show what your class is like.
I would say if they aren't willing to pay even $5 for a trial, and aren't satisied with a prerecorded video preview, they probably aren't worth spending a lot of time and effort on, as they are probably just looking for free content rather than legitimately wanting to test your services.