[¯|_(ツ)_|¯]

I think that you should sit down and talk to your mother. Let her know that you understand that she may be hesitant to allow you to work on weekdays due to you getting tired a lot and because you are still in school. But, also let her know that maybe she should give you the opportunity to try it before ruling it out completely. You can try to make some sort of compromise with her where you are able to work some of these weekday shifts, but if it starts affecting your schoolwork, you start to lose energy, or your mental health worsens, you will reduce the amount of hours you work again. That way, you will have the opportunity to at least try and work these hours but recognize that if it gets too much for you, you'll have to reduce your hours again.

Legally I'm not sure if she can physically change your hours. Right now you are still a minor so she may be able to prevent you from going to work, but you're going to be a legal adult soon. This is something you can speak with your manager about, and maybe follow some of Robin's suggestions as to approaching the manager and telling them that without your signature, any forms should be disregarded.

I wish you the best!

-Dez