Your marriage (if you ever marry) is your life, not your leaders'. I know it can be hard to contradict an authority figure, particularly if you are under 18.
So go slowly. Date only who you want to date, because you are attracted to that person, and they are to you, and not because someone's brandishing a calendar or a Bible at you.
As for this new girl, be your nice, polite, friendly self. Will you date her? Maybe. But that's not likely to happen unless you are friends first, so make friends! And, by the way, be kind and friendly because that's the right thing to do, and not because you have a future dating agenda.
Because someone new has come into your community, my gut tells me your community isn't the only place to find people in your denomination. Perhaps you could go to college at a religious school or in the area where there are other folks who are of the same denomination as you.
The 20, 50, one thousand or however many youth there are in your youth group are not the only appropriate future marriage material for you.
There is no sin and no crime in waiting for the right person, rather than settling for someone due to outside pressures.