Security and Privacy Setting Problem

I have a MacBook Pro, school issued, and I have an administrator account. It is running El Capitan version 10.11.4

For a few weeks, and I'm not sure exactly when it started, it has been logging out automatically. In addition, when it logs me out my password is not always recognized as correct, so I have to restart to log back in. I finally found the setting in Security and Privacy that is checked for log out after 30 minutes of inactivity and unchecked it. However the setting changes I make do not stay, whether I uncheck the box or change the amount of time before log out it returns to that 30 minute log out setting. As a teacher in a busy class I'm finding this increasingly frustrating. I can't afford that lost time if I need to show the class something on the board or find a resource. Can anyone help?