Yes, if I understand what you're asking, that should be quite feasible. It sounds like what you need to do is proxy ARP for your public scopes upstream, then push them downstream to each campus with a series of 1:1 NAT rules.
Regarding removing the pfSense nodes downstream, I'd be cautious about that. It's a good idea to have something segmenting the schools off from eachother downstream. Keep in mind that students are often brighter and more capable than school faculty, especially in technical matters, and should not be underestimated.
One final piece of advice would be to stage as much as you can before it goes live. Also might want to run some serious torture tests on the hardware/software stack you plan to deploy, ensuring that:
The hardware is reliable and won't be a bottleneck for the amount of traffic you're expecting, + predictable growth.
pfSense / FreeBSD is reliable enough on your hardware stack, and has all the features you need.
You know exactly what to expect in terms of configuration, backing up and recovering configurations (if the interface names don't match you're in for a fun time), etc.
Regarding the stability of pfSense / FreeBSD, I ran into some rather serious issues myself which essentially blocked me from deploying pfSense 1.2.3 in an overly-hostile environment. YMMV of course, but here's the record of my endeavors for reference: http://forum.pfsense.org/index.php/topic,24337.0.html