Classroom courses

If I buy a course and only 15 kids are added will the other 15 spots carrry over to be used whenever or will I lose those spots at the end of the year?

If you purchase Tynker Classroom, which includes 30 named licenses, you can use it over the life of your account. So if you use it for 15 students this year, you will still have 15 licenses for next year. If you purchase Tynker School, that is an annual license.