While searching for thermals 5 miles east of the airport, the pilot realized he had insufficient altitude to glide back to the airport. He elected to land in a school soccer field. During the approach, the pilot noticed a 5-foot high hill in the middle of the field. He decided to land on the other side of the hill. After touching down, the pilot applied 'moderate braking.' The aircraft went off the field and collided with an embankment next to a road.