I have been able to cache el-capitan and users can go to self service and install it. Is there a way for me to force the macs to reboot and install it on its own we are trying to get everyone to be on the same OS and its been a year and half and I still have users asking me what self service is because they didnt pay attention at our training.
So I've done this recently with Yosemite and it's the same with El Capitan. I have a script that cached the installer and then ran it during off hours (10pm). When it runs it, it really just installs the "OS X Install Data" folder on the root and then blesses it to reboot into it. I noticed that sometimes 90% of the machines just boot back into their normal OS. So make sure you add a restart option in the policy to restart to the "OS X Installer" partition. BUT, since your doing this off hours, have another policy looking for machines with that "OS X Install Data" that are still in your old OS (i.e machines that didn't reboot into installer partition) - and force them to reboot into OS X Installer partition. I find that most machines needed about 2-3 reboots to get into installer partition.
Yeah you can make a policy that runs the cached installer at a different trigger event like login, network state change or checkin. Under action for the package, select install cached and make sure under restart options it "restart if a package or update requires it" is selected.
Although this will disrupt any work your faculty are doing and probably won't make them very happy, I've considered just doing the same with our faculty. They refuse to take action and update!