In short, I think the idea of a centralized call center, with at least 1 "public" number, and 1 private number is ideal. If an area has more than 5 students coming from it, get one of your partnering organizations to set up their own little facility, or have someone volunteer from there. That way, you call your call center, and they call the little local facility, and then that person calls the local area families. The system would work in reverse too. That way, the most amount of communications can occur in the least amount of time. Consider alternatives like email for these types of communications also.

You have not mentioned much else of what you are planning to do, and probably understandbly. However, a communications plan only serves to enhance the overall effectiveness of any plan. A plan should not rely on electronic communications to be effective. You are relying on a satellite phone with batteries. What happens if it's near freezing, will the battery work? Did the incident occurr during a period of high solar activity, resulting in interference, or worse, the satellite being fried? The combined failure of a satellite, and grid power is likely at higher latitudes. Consider supplemental and alternatives to your communications plan, such as FRS, GMRS, Amateur Radio, commercial FCC license, and word-of-mouth runners. Also, you should make sure that "preset orders" are in place. "If you cannot communicate with anyone, you are to proceed to the McDonalds at Main Street and Flower Avenue in XYZ town". That way, if someone doesn't get in touch, others can relay the intended future location of the others.

The emergency/disaster will also be these students emergency. While you have a legal responsiblity for them, that doesn't mean you cannot include them. The students need to feel that they have some sort of control. The easiest way to do this is to create a buddy system. Someone they will always be with, and can rely upon. Second, because of your numbers, you need to implement a team heiarchy structure to manage all of your staff and students. Consider using the FEMA National Incident Management System as a guide. Adapt it for your use. From your maximum numbers, you have 1 person in charge, and the rest of the group divided in half. 2 people in charge of a half, and 4 or 5 teams underneath. Each of these teams is comprised of 2 staff, and up to 13 students (6 to one leader, 7 to the other). It may be more effective to transport 15 people (15 passenger van!) at once, than 120. This also follows the span of control of 5-7 people. Each staff member should have communications of some sort.

Each participant in the plan should have instructions for them. For example, your specific evacuation plan need not be known by parents, although they may like it, but they should know you have one, and that you plan on rallying at point A or B. Each student should have a emergency card that has their name, next-of-kin (not always the same as ICE!!!), home address, important phone numbers for the local area, and any other relevant information(maybe pre-planned rally points).

Some will probably think this is all overkill, but chaos and a lack of planning is a SERIOUS problem during an emergency. Not having a plan is a bad idea...

Edited to add:
The previous post has alot of good ideas for considerations, you need to know what you are going to do first, before you try and use communications. Also, the above is my suggestions, and what I would do. Do your best, and plan for everything. <img src="/images/graemlins/smile.gif" alt="" />

A good communications plan is one that ensures that the "orders" and "information" get through all the time, everytime. So, plan for failures.