I'm trying to find a kids movie from the 1980s, about a group of kids (there was at least one tomboyish girl) that had a secret underground clubhouse. (I believe they entered it through a sewer). They either skated or rode bikes, (I was really young, I can't remember). I think they had a battle with other kids and they needed to fight, so they enlisted the help of an old man to train them. This is driving me crazy, I've spent so many hours searching for it. Google is not helping!