At 6-5, Middendorf was a terror on the ice for St. Joseph of Montvale. He helped St. Joseph to a pair of Bergen County League titles. In addition, St. Joseph won the 1984 state championship with an incredible 26-0-1 record. That season, Middendorf had 38 goals and 35 assists.
Middendorf was the first scholastic player from the Garden State to make it to the National Hockey League.