An administrator can change the following settings in Administration > Site administration > Server > Session Handling.
Once someone logs in to your Moodle server, the server starts a session. The session data allows the server to track users as they access different pages. If users don't load a new page during the amount of time set here, Moodle will end their session and log them out.
Be sure this time frame is long enough to cover the longest test your teachers may offer. If a student is logged out while they are taking a test, their responses to the test questions may be lost.
Most of the time, you can leave this blank, unless you are running more than one Moodle site on the same server. In this case, you will want to customize the name of the cookie each Moodle site uses to track the session. This enables you to be logged into more than one Moodle site at the same time.
Note: If you change "Cookie prefix" or "Cookie path" you will need to login again as the changes take effect immediately.