In the 15 years since I first used a Mac, I've sent text and photographs in every concievable file format from Macs to print services, large CMYK presses, individual end users with bubblejets, et - and I've never had a rejection issue because of a platform incapatability.
That's a piss-poor printing service if they can't accommodate the industry-standard platform for most graphic arts and imaging fields.
What kind of a creative/ design firm doesn't run primarily on Macs in the first place?

Are they run by the Texas school board?