Most long-distance calling cards do exactly this, and can be had for pennies a minute

If this is truly a "large school," basing your network on a single server is such a bad idea it is almost criminal, and implementations like this are what give Windows (and Linux for that matter) a bad name.

I question why you have separate networks for students and teachers, but that aside, why in the world are you giving your network a single point of failure like this? One of Active Directory's strengths is its ability to use multiple servers to achieve redundancy. Why are you running 2 domains with only one DC, and why would you design a new implementation with a single DC/LDAP server/whatever? What happens when that machine has a catastrophic software/hardware problem?

Also, change for the sake of change is a poor idea. If you have a legitimate reason to say that $FOSS_LDAP_SERVER is a better fit for your environment, that's one thing, but by not even considering that AD *MIGHT* be the best fit for your environment, you are doing your employer and clients a disservice.

Hire a consultant or someone that knows what they are doing - regardless of which platform is picked. From the question, it sounds like you don't.