I assume you mean that one particular girl is always in the middle (otherwise the question is silly).
6 ways to choose the first girl (7 minus the one that is always in the middle) times 5 ways to choose the second girl (7 minus the one in the middle minus the one that was picked first) times 4 (7 - 3) times 3 times 2.
In other words, 6!
John
My calculator said it, I believe it, that settles it