BE-35 flight Instructor with student reported a gear up landing occurred when the student inadvertently retracted the landing gear during rollout.

Narrative

Gear up incident report. The pilots on board during the incident were myself; [the instructor]; and my student. I am a certified flight instructor; with instrument airplane privileges. My trainee is a private pilot with complex (61.31(e)) and high performance (61.31(f)) endorsements. We were flying a small; low-wing retractable gear aircraft. The following are the events in chronological order that lead up to and include the incident itself. The plan for the day was to do standard instrument training while on VFR flight following. We started the day at XA:00am and spent approximately 2 hours on the ground briefing our flight which was to include: VOR tracking; VOR intercepting; VOR approaches; and ILS approaches. We took off out of ZZZ at XC:45am. We flew the VOR-A in ZZZ1 at about XD:15pm and went missed approach. We then continued to ZZZ2 and flew the VOR XX with the published missed approach and held. We left the hold at approximately XD:45pm and continued to ZZZ3 where we were vectored by ATC to the ILS XX. After going missed approach we proceeded direct to ZZZ4 and once again got ATC vectors for the ILS XXL. In ZZZ4 we performed a touch and go landing in which I thoroughly instructed my student to leave his hands on his side of the plane and do the takeoff as I took the flaps out. We did this for efficiency of the flight and do not regularly perform touch and goes in this airplane. At XF:10pm we were cleared through Class B airspace up to 8;500 feet. At XF:15pm I got a call from the trainee's brother; asking to be picked up in ZZZ5 with his other brother. We calculated our fuel and if we could make the trip and still get to ZZZ. We decided we would do that. I had the trainee make a full stop landing in ZZZ5 at approximately XF:50pm. We picked up the brothers and proceeded to ZZZ. At approximately 25 NM south of ZZZ I cancelled our VFR flight following and we continued VFR on our own. We briefed the ILS XX into ZZZ and noted that the wind was 230 at 07 knots. I asked if he wanted to try a tailwind landing as I wanted to expose him to a different condition. I told him how on his landing he might float a little longer; but how we had plenty of runway so it wasn't going to be an issue for us. It was clearly understood by all parties that this would be a complete stop. Upon reaching the ILS XX we tried to engage the approach mode of the autopilot to see if it would catch the glideslope. It failed to do so and we continued the approach visually straight in.At about 7 miles south of the airport we briefed the GUMPS checklist (gas; undercarriage; mixture; prop; seatbelts). We had a green light on the landing gear; the trainee pushed the mixture full rich; rolled the propeller in to high rpm; and continued his approach to landing. With full flaps; gear down; and him on the flight controls; we entered ground effect at about 75 knots. The nose was just slightly pointed left of centerline. Because of this my vision was more focused outside in front of us; as I talked through the landing. I told him to add a little more right rudder to keep the nose straight as we landed. We touched down on the main wheels first then brought our nose gear down to the ground. As we were rolling on the runway with all three wheels; it is my understanding that the trainee took the flaps out. My eyes were looking outside in that moment; and when I brought them back in the plane I saw the trainee's right hand lifting the landing gear lever to the up position. As the landing gear warning horn went off; I shouted in the moment not to do that and tried to put the lever back down. As I got it back to the down position; the gear collapsed and we slid along the runway for about 400 feet. The propeller struck the ground and all three blades were bent and we came to a complete stop. This occurred at XG:15pm and there were no injuries.Throughout previous training of this plane I have informed the trainee; to never touch the landing gear lever while on the ground so a mistake like this isn't made. I mention that because the plane the trainee learned in; a single engine high wing; has flaps in the same spot as the landing gear lever of this airplane. Out of habit; the trainee reached to the right side of the plane and lifted up as if it was the flaps in the high wing aircraft. We have also gone over the importance of the checklist and using it after we get off the runway. We could have prevented this incident by simply coming to a complete stop and getting off of the runway before touching any switches. After the incident occurred; I pulled the mixture; throttle; and propeller governor all to idle. With the avionics still on; I made a radio call to the ZZZ UNICOM and told the airport worker; to shut the airport down and that we were stuck on the runway with no way of getting off. I then shut off the avionics; ignition; battery; and alternators. I exited the plane through the door; with Person A and Person B following behind. After breathing I went back up to the plane and made sure my student exited too. I then gathered the headsets and personal items and selected the fuel selector valve to OFF. After looking at the damage it was clear to see the landing gear was crushed; the propeller was bent inward; the cowl flaps were damaged; the pitot tube was smashed inward; and the nose gear took significant damage. We took pictures of everything before leaving the plane and runway.