I don't know of a school or group that offers free lessons, but I do know that a lot of racers--local club racers on the East Coast at least--are always looking for willing hands, and I can't think of a better way to learn the ropes as long as you're agile, a good listener, and eager. My cousin has welcomed countless young people who went on to become part of his crew and then to own their own boats. And, let me tell you, if you could find someone like him to teach you, you'd be fortunate. He's a marvel.
Also, at the docks a lot of single handers--daysailers--would love to have someone on board to play with them. Put your name up on club or dock bulletin boards, chat up various sailors if you see them messing about on board. If you're willing to help them do a little boat work, you might be able to parlay that into a lesson or two.
You don't have to have formal lessons from a school. I've been sailing all my life, and my best teachers were my aunt and the racing (3rd) cousin.