What should i do?

If you were older I would say be true to your g/f but at such a young age, you need to have fun with different girls and not be tied down to one. At 15, you aren't ready for a serious relationship. There will be many girls you are attracted to and want to be with before you find the one you will want to settle down with.. Do you have this girls number or have any way to contact her? Be with the one you want who also wants you. See what happens... maybe you should let your g/f know you're not ready for anything serious.

Dardar94 ,  Be honest your with current Girlfriend , break it off with her ,tell her you are not ready to be in a committed relationship .  Then you are free to pursue a possible relationship with this other girl without being dishonest and disrespectful to your current Girlfriend .  And at 15 you are not ready for a committed relationship , but this does not mean you should not accept the responsibility you have to your current Girlfriend to be honest with her .