This may not be true everywhere - depending on local laws.
But in training as an insurance adjuster in the USA, we were firmly told that whoever waved someone into the road could be held legally responsible for any ensuing accident, if they could be tracked down.
I've heard of one guy who was asked by the police at the scene of an accident about why he waved someone in front of him when traffic was NOT clear - he said that he was trying to get fresher air in front of his face after a "gas leak" (the kind some people try to blame on the dog)...he hadn't been waving at anyone!
I've also been an adjuster where someone claimed to have been waved into traffic while they were trying to make a left turn at a similar situation. But since I had driven by that high school before moving to the place where I got the job as an adjuster, I knew that the left turn lane had been cemented in with a raised "hump", the hump painted yellow, and "No Left Turn" signs posted on the hump and the exit from the parking lot to the highway - because it just wasn't safe to try to make left turns out of the high school. The name of that road was "the Dieway" - so making right turns wasn't much safer. There was a LOT of traffic there....