TFA says they want a multiplatform security solution with more than just AV but also antimalware, URL scanning and probably stuff like page source scanning for malicious JavaScript and the like. A Linux or Mac is less prone to malicious executables for now, but what about later when more show up? Just because they're the minority in the OS market, they're growing in popularity and are beginning to come to the attention of the seedy side of the Internet. A general user with admin rights will blindly enter their username and password to confirm the installation of whatever flashy malware toolbar or cursor icon changer that catches their eye, regardless of what OS they're running.*
Also, phishing email and websites are fairly OS agnostic and users will enter their bank or credit card info onto fakebank.com's website if given a chance. A URL scanner/blocker that is centrally managed can help minimise the impact of common known phishing sites and also help in targeted phishing attacks customized to the organisation - common ones like email from support@yourschool.blah saying something like "due to a failed mail server maintenance we require you to login and reset your mail credentials here at website blah". Just because you have Mac or Linux users doesn't mean they're immune to social hacking.
Speaking of central management, having all your endpoints reporting security information back to one central product makes security easier to mange for you as an IT admin. If you can cleanup infections on Windows remotely, that's great. But now you get reporting of whether Macs and Linux computers are receiving infected files an clean them before they're passed on to Windows computers. Plus, these security suites may also include a host based firewall program so now you can control that in the same console as well regardless of OS.
Additionally, due to laws or regulations such as privacy laws or PCI compliance or whatever, some computers might be handling personally identifiable information (student numbers, addresses, birthdates, grades, etc.) and Data Loss Prevention mechanisms must be in place and auditable. Plus do you really want to set a separate new central mangement and reporting solution for all this stuff for every OS?
Having worked with several of industry leading solutions I'm not sure if any of them are really fully cross platform - that is to say, not all the functionality that is available on the Windows platform is available on other operating systems, but if you want vendors to sit up and take Mac and Linux on the desktop seriously then movements like these are needed; saying that for my organisation, Mac and Linux are just as important as Windows and if you want my business you, as a vendor, need to support them equally. We should be praising that the Tamanian Dept of Education is promoting minority operating systems to be taken seriously.
*I know that the solution is not to give them admin access but Windows is very secure if locked down properly as well. Also since this is the education sector, the IT group probably isn't given the mandate to lock down computers anyways so users very likely have admin rights. Also being the education sector, there are probably multiple IT groups in lots of geographical areas and most are probably under resourced and underfunded.