I know it's a little moot, but, it would have been a lot simpler and easier to just go 'Hey teach, this kid behind me keeps smashing their feet into my back, they've been doing it for the past 20 minutes.' Or turn around and don't confront her and be silent about it. Confront her and be obvious. "Hey, you wanna stop kicking my back like you're a 10 year old on an airplane, grow up."