You probably won't be able to change the look of the original structure to match the new addition. In other words, you can't add a stucco addition and put stucco over the brick to make it all match.
I would ask about their requirements before you start the design process.
I would also ask about the zoning requirements for having a school in a neighboorhood. There will be issues of parking and signage that will come up.
"What rules apply to renovating in historic neighborhoods? "
The City Building code, Compliance department has the final say. These are the people I would look to for answers.
This is one of those times that you need a "real" expert in this field. Greg Swann knows the Historic district better than anyone, here is his website http://distinctivephoenix.com/