I have 8 Mac Mini's all running Windows 10 as a piece of software I required to run only supported Windows. The IT assistant who managed the installation of Windows regretfully did not use Bootcamp or similar software so the computers operate just as Windows machines and use a standard Windows keyboard. I now want to get them all back to Sierra OS to I can use Bootcamp properly but despite creating an installer USB drive using my own Macbook Pro, I cannot get the computers to boot to the drive and therefore reinstall OS X over Windows. I have tried using Advanced Startup options through settings and selecting the USB drive but Windows still boots normally. All support threads I can find online relate to either installing OS X on a PC or restoring OS X on a Mac, cannot find any advice on reinstalling OS X on a Mac running Windows 10. Any support would be gratefully appreciated.