Can any of you remember studying a fiction book about WW2 at school in the 90s?

I remember that the book was about a group of kids who saw a German fighter plane get shot down. They made it to the plane first, they helped the injured pilot and took the MG42.

I think one of the characters was a girl but acted like a boy and the main character finds out that his best friend is actually a girl - that could be wrong though.

Any help with the title would be greatly appreciated.