This student was able to keep the back relatively upright but still struggles to hover without touching the bar.
The problem is at the knee joint: in the before picture, the student was squatting too low and theres a lot of flexion in the knees, this result is reduced pressure on the calves and hence board not edging enough.
After: I simply instructed the student to straighten the knees slightly, immediately the calves press against the boot more and hence edging the board more. Successfully hovering without touching the bar