A sister would like to ask: In an online school, if a child is more than 5 minutes late and is not allowed to join the class, while parents are still required to pay the full monthly fee, is it permissible for us to pay for something we did not receive?

It's not about paying for something you didn't receive; it's about not abiding by the rules and being prevented from participating because of that. I believe this would be stipulated in the contract you agreed to, and you didn't keep up your end. All you have to do is be on time.