With a simple ticket system. On day 1, you buy lesson coupons from your referring teacher. For each new lesson you take, you give one back. Coupons work equally for private, duo and mini-class lessons.
1 coupon = 1 lesson (1h30)
Flexi-learning. Once you have bought your coupons, you can use them the way you want - spending them on private tuitions, duo tuitions and mini-classes at your leisure - and when you want. You write your own agenda and take up lessons at your own pace.
Here is an example on how a student might choose to use 7 coupons on a given week: