If a student does not show up for a first time trial, you are allowed to charge the student for that FREE trial. It is up to your good judgement if you want to offer it again.
If you have a student that habitually misses classes it is customary to charge for that class. If you have a teacher relationship with a student, try to communicate with them about their absences. Again, you are the teacher, and you should be paid even if they do not show up. Set your rules with them upon the first class.