An average good high school 4x400 is 3:20. (Equal to a 4:20 miler)
A 3:20 4x400 requires some talent but not out of this world talent. 4 dedicated hard working 400m guys should be able to go 3:20.
3:20 4 x400m is NOT the national average. If we were to search high school 4x400m records or converted high school 4x440yds records, the average high school record would be between 3:25 to 3:27.5