I'd frown on a P22. Small size and weight plus it's relative inaccuracy makes it a poor tool to get beginners into handguns.
Although the goal is not to teach combat or full-size DA-semi pistol ergonomics, a previous poster has a valid point.
The first .22 I ever shot was a ruger single action. Such a revolver is clumbsy to load/unload however, it in the process it gets people comfortable & confident with handling a pistol when they spend more time loading/unloading than firing. Also, in the process, you have less of a chance of the trainee making a accidental discharge.
In the semi catagory, any Buckmark or Mark II type would probably be best.
Ideally, I think that a three step SA revolver to DA revolver and a graduate to a Semi defines a ciricullum that keeps interest and builds a pretty broadd base on handgun functionality. I understand that having three training pistols is probably not feasible, just my .02.