She says that it will distract me at schoool because I will play with it (im 15)

Also, it will break teeth and fillings, mess up your speech, cause mouth infections and bad breath, and interfere with eating.

Maybe you should wait until you're old enough to do it on your own.

Just bribe her. Tell her if you she lets you get your tongue pierced you'll do more chores or try harder in school. Its just something to bull**** her, you don't actually have to do it, cause you'll already have your tongue pierced. :)