I agree with what your principle's concerns and Blake's suggestions. I had a similar idea, one that I actually carried out: I did a clinic at the local Boys and Girls Club, which ended up being pretty successful. I'll share my thoughts and lessons learned from this event.
I think a small demonstration with an associated presentation at a local PTA or school assembly would do wonders. Having children who are currently lifting (be it from your club or from a nearby club) with those who've been in the sport and benefited from it (the older crowd, be it weightlifters or other athletes), you can show not only the movements being correctly performed, but present how these movements are taught in the early stages to better develop athletic performance through the testimonies of those that have been in involved with the movements.
It will be critical to show that you have a solid understanding of athletic development, that at your potential audience's young age (pre-pubescent middle schoolers), you're trying to ensure that -- above all else -- they have proper technique and not trying to unjustly push their weights. At that age, they should still be in a stage of development suited for (dare I say) general physical preparedness, i.e., they're still trying to learn coordination and continuously adapt to their ever-growing body. That's not to say that some won't show extreme potential and advanced development, but they should be afforded every opportunity to continue to develop all areas of physical aptitude rather with an opportunity to enjoy weightlifting.
I also presented my own experiences and how I wished I found the movements associated with weightlifting sooner in my athletic development. As a wrestler, my development could've been that much better - I would've been stronger, faster, and more aware of my body. While wrestling is very individual and mentally demanding, I could've also mentally developed myself a little more had I ever competed in a weightlifting meet or two.
This could be a daunting task, but with a little rehearsing and refinement of the material that you want to present, you can get this down to about 20-30 minutes in order to ensure that you aren't losing the parents' interest. They should have every opportunity to ask questions without feeling restricted by time.
Here's a video of a local coach and lifters in my area: https://www.youtube.com/watch?v=QVP3Fb-NMSc
Here's just a video of children lifting at a competition. I think it would be important to note that they're not lifting anything heavy, but that they're focused on being technically proficient: https://www.youtube.com/watch?v=TX8ClqbO5OI
I hope this helps,