If you really want to cull a bunch of crappie, then a fishing derby is an excellent idea.
Do you know any Boy Scouts at school? If you had two scout troops come out with some leaders that want to teach fishing, they could catch a bunch of crappie when they are biting good.
I would make sure to determine what lures/baits are successful at catching your walleye, and insist that they do NOT use those baits. Also don't allow any live baits that you don't want in your pond. There is always a chance that a few will live (but probably unlikely).
I would also enforce a rule to throw back any fatter/heavier crappie. If they are mostly catching crappie that are X" long, then you might tell them to cull every skinny crappie within an inch of that length to either side. Or, if they all look skinny, then have them cull every single crappie they catch. I don't believe you can fish the crappie out of a 22-acre pond unless you also have a mass of predators.
