Disable auto download of high sierra

We are a school and the "install macOS High Sierra.app" auto downloads on of our Macs. I have found this to happen over the internet if the Apple Update Server setting is not set on the devices and then the Local Apple Update Server if the device is set to go to a local Apple Update Server. The local Apple Update Server has downloaded and lists "install macOS High Sierra" in the Updates. I can Disable the update or even Remove it in the Gui on the Server It will automatically Enable it self. I have had to turn Off the local Update Server to stop all of our devices from downloading this 5.69GB file in the background and clogging up our network. Is there a way on the devices to turn off this auto download? I have already disabled "Automatically Check for Updates" which turns off the download of updates and installing the security updates as well. As I have mentioned we are a school and having all devices(about1200) downloading a 5GB file at the same just cant happen and brings our network to a crawl.