It recently occured to me there could be another option with these vouchers.
Suppose the kids came in to be tested every two months and as long as they remained above average they don't have to come to school. Tell them what books cover the material that will be on the tests and let them do what they please.
If the kids have passed all of the tests at the end of the school year and haven't spent the vouchers with any schools or tutors then they can cash them in and keep the money. I bet you would find a lot of kids are suddenly very good at educating themselves if you give them decent books.