Listed a few Ideas that may help. However, ASK YOUR MOM BEFORE DOING ANYTHING !!
Here are some suggested home remedy that may help to whiten your teeth.
1)Brushing with Lemon Juice
2)Brushing with Baking Soda
3)Brushing with Hydrogen Peroxide
4)Brushing with Salt
5)Rubbing Strawberries on your teeth then Brushing off.
However, all though the above may help whiten your teeth they can do damage as well. They contain sugar and acids and are abrasive, all of which can damaged your enamel that helps protects your teeth from tooth decade
There are many different types of tooth past that have whitening as well as mouth wash with whitening this may be another alternative.
With that being said. I need to add. I would wait until such time that you can go and speak to a dentist. The remedies listed has issues that may make things worst off then just a small stain as you discribe.
Even though you are starting school, I bet most people will not even notice a stain on your teeth unless you point it out. If you do run into someone that is going to pick apart something like that, and then make a comment on it. Well, keep in mind who that is coming from. More then likely it is someone who has low self worth and needs to find flaws others just to feel better about them self. If you do not make a big deal about the small stain you have others will not either. Then if you are still bother by the stain when you are able to see a dentist you can have them take care of it.
Well I hope I have help