The certificated flight instructor (CFI) reported that the student pilot successfully completed crosswind landings at a grass airstrip prior to making a landing on runway 31 back at the departure grass airstrip. The winds were from 360 degrees at 14 knots with gusts to 20 knots. The CFI reported that when the airplane was about 30 feet above ground level, the airplane encountered an “unexpected strong down flow of wind (wind shear)” coming over the tree line on the northeast side of the runway that caused the airplane to hit the ground hard. The CFI took the controls prior to the airplane hitting the ground. The airplane bounced and the CFI attempted to recover the airplane, but “didn’t have time to explain to the student what was happening.” Both pilots were on the controls when the airplane touched down the second time. The CFI reported that both brakes were “locked” and the airplane skidded for about 75 feet before the airplane nosed over. He reported that he could not counteract the student pilot’s brake input.