I know in the real world many rural counties have one high school for the entire county. Can the same thing be applied to the Simcity 4 deluxe game if I make a large city into a rural setting? The same thing applies to the police department the HQ is usually at the county court house area for the entire county.
Unfortunately, the school radius can't cross city boundaries, but it sounds like what you need is a school with a radius the size of a large city tile. I don't know if there even is one...
While I can't give you any specific examples, I know that there are many custom schools that have an increased coverage radius over that provided by the default Maxis ones.
Another alternative (if you are comfortable using Reader to mod building statistics) would be to take any school building you like the looks of and mod the relevant properties to increase the radius.
I think there are also some modded all-in-one civic buildings that provide police, fire, elementary & high school (maybe more) coverage in one.