Actually, the stunt was not rehearsed per se. The set weighed over 2 tons and was a one-shot deal. The cameraman refused to watch, he closed his eyes but kept the camera going the whole time. It's a testimony to Buster's attention to detail and his confidence in the structure of his gag as well as the abilities of his crew in constructing the house front. It had to be the "real thing" in order to fall completely flat and not warp.
And contrary to the instructor's comment about there being a foot of clearance, there were only 2" on either side of Buster's shoulders according to Keaton himself. There was a nail in the ground at the precise spot he needed to stand.