While seated behind her on a low chair with wheels, I grip her knees. Then I either move her legs forward or wait for her to move the leg independently.
The method is from M.E.D.E.K., a type of physiotherapy which I highly recommend.
PS We have been invited to meet privately with C.'s teacher tomorrow in order to fill him in on what we do with C. at home. Pray I bite my tongue and keep my seething in check.