A Flight Instructor reported that the left main gear collapsed upon landing.

Narrative

The flight started normally; pre flight was completed and nothing abnormal was found. Start; taxi; run-up and take off were normal. Throughout the flight I had the student demonstrate slow flight and power-off stalls. It was a relatively short flight because the prior student returned the aircraft slightly late; but I don't currently have the exact length of flight time; probably ranging from a 1.0 to 1.3 hours. Returning back to the airport; we decided to finish with a few laps in the pattern. All laps were touch and go landings and all checklists in all phases of flight were completed. I don't recall any anomalies during the first two landings that would lead me to believe there was an issue with the landing gear. On the accident landing; downwind; base and final legs; altitudes and speeds were normal and the wind at the time was variable at 3 knots. Everything up to the landing was normal to me; my student flared; and the landing was firm. The left wing was abnormally low as we continued to decelerate for a few seconds and then the left wing dropped down onto the strut housing and came to a stop shortly after on the landing runway. After coming to a stop; the landing gear strut was observed to have completely exited the strut housing and was under the left flap. Tower was notified and the aircraft was safely removed from the runway and put in the airport's maintenance hangar. No persons on board were injured.