ANSWER:

If you are taking your fees on monthly basis and they did not exclude the days they are going for vacation and still pay you, this is a gift from them. 

Those who stop paying because they did not attend with a prior agreement with you, this is fair and just. Charging them while you know they will not attend would be taking their money unlawfully especially when they had told you in advance.

QUESTION: I teach kids online. They pay monthly in advance. sometimes some of kids go on holiday 4 a month & I still charge. Some parents don’t mind paying when absent & some don’t want to pay, so is it ok if I charge some & don’t charge the ones who don’t want to pay?