Maybe it is just me, but what exactly does being boyfriend and girlfriend at 15 mean? In the US, you can't even drive at that age, so it is hanging out in the halls at school? Maybe getting a soda at the drug store on the way home? Holding hands at the school dance?
To me, most very young relationships are not much more than very good friendships. I think sometimes we want to grow up too fast and feel a need to get into a "relationship" when really we are just comfortable liking a lot of people and spending time with a lot of people.
Now, with that being said, there is no reason to avoid the other girl. Be friends with her. There is nothing wrong with that. You never know what will happen in the future.
Good luck.
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