I took over a moderate Mac network here and then added to it. Had a few issues with OD integration but nothing a bit of research didnt get to the bottom of. We now have almost 100 Macs running VM Fusion so that all students have the best of both worlds. Now we know how to controll the XServe and how everything works we wouldnt be without it.
Only justification I can see for Macs over PCs (and I'm a Mac user meself, but with 20 years of commercial PC hardware development and support in my back pocket) is for very specific apps that are damn good at what they do that are only available for that specific platform, eg: Logic / MainStage, Garageband, BoinxTV, FinalCut, & xCode (if you want to write native iOS apps for the appstore). These are all far superior to the equivalents on PC in terms of value for money and time required to produce quality output. It's a balancing act... there's nothing Logic & Garageband can do that Cubase can't. There's nothing Mainstage can do that Steinberg Sequel can't. There are appropriate apps for PC that cover those bases. Cubase on a PC = Cubase on a Mac (other than the soundcard uses CORE audio on Mac vs ASIO on PC... and CORE tends to be better at working with generic hardware than ASIO, thus ppl appear to see less latency on Mac when using Cubase - solution there is to choose your soundcards for your Music dept based on ASIO2 driver availability). Adobe CS5 / CS6 on a PC = Adobe CS5 / CS6 on a Mac. Office on a Mac = Office on a PC. There are no significant differences or benefits other than the IT dept will need to know / learn how to roll out customisations, apps, restrictions etc via a different method to what they're used to.
That said, there may be cases where a Mac can be justified over a PC (eg: iOS app development), but the majority of those scenarios are single members of staff doing specific roles... in which case sliding in a single one-off Mac device is relatively simple and painless.
The issue as I perceive it (based on what I've heard from various senior leaders of a few places) at the moment in current economical climate is that parents and prospective pupils are more likely to go "wow" when they see Mac devices on an open evening, and on that basis are more likely to attend the school. Ditto for sixth forms. In the case of sixth forms, the money that the establishment receives per head is significant, so anything that can be done to draw students into enrolling is worth doing. I have overheard it said that media course at establishment X and same course at establishment Y are identical in terms of results/performance of pupils, but establishment Y has Macs not PCs, therefore our child will be attending Est Y. It turns out Est Y has 30% higher enrolment on that course, and many put it solely down to the pull of the Macs on open evening, and that translates to a lot of cash from government for Establishment Y that Establishment X won't be getting...
So in some cases, the reasons for wanting Macs isn't at all technical, and is more grounded in marketing and PR... and when it comes to sixth forms, Macs can be quite justifiable in terms of return on investment if it gets a student enrolled.
S'all about appearance *shrug*