I think your mothers concern are the following:

- You're loosing money and can get in debt.
- You're spending too much time playing poker and are going to perform not as good anymore in school.

If you can convince your mother in both points she will probably let you play as much as you want. How to do it:

- take a note of all your wins AND losses. You may fix a limit on how much you can loose each month before you stop playing. Maybe you have some loses in a row and must stop for some time, but you should stick to your agreement at any point. This also helps you to stay out of trouble and may improve youre wins.

- the second one is keeping good results in school. When she sees your doing fine she will also be more likely to allow you play more. And especially mathematics will help you improve your play.

And my personal note: Be careful as you dont have a regular income loses are much harder to pay for.

Good luck
Dancer