Shane Needles of Nashotah died last July while he was on a field trip. He went into the river at Riverside Park and never resurfaced.
The lawsuit filed July 9 alleges Shane died because of a lack of supervision. WISN-TV reports the lawsuit says there were not enough adults to watch all of the children.
Taekwondo Academy told WISN: "We're horribly sorry that this even happened, and we wish the family well."
Family, friends and classmates held a vigil for Shane Friday night, and released Chinese lanterns into the sky.
Shane was a student at Lake Country School.