The flight instructor was on an instructional flight with a student pilot under Title 14, CFR Part 91, and decided to land at a different airport prior to returning to the departure airport. He said while above the airport where he intended to land, he received a report from another pilot that there was 4-5 inches of snow on the runway. He made a successful soft-field landing, and said he taxied back to the threshold, and prepared to demonstrate a soft-field takeoff. The pilot said the airplane accelerated slowly, and did not reach rotation speed before running through the runway end markers. He said the airplane continued down an embankment and came to rest in a creek. He said there were no known mechanical problems with the airplane prior to the accident, and that the airplane sustained damage to the wings and fuselage. In a written statement dated November 18, the pilot wrote that the accident could have been avoided by not landing on a runway with more than an inch of snow.