Students should behave reasonably and not cause undue disturbance to others during laboratory sessions (and other times for that matter). Nowadays, one cause of disturbance can be students answering their mobile phones and shouting loudly into them. It is reasonable for you to ask them, politely, to either stop their phone call or leave the laboratory.
Another source of disturbance can, on occasion, be students playing multi-user games and getting over excited. Again it is reasonable for you to ask them to leave. In any case, game playing is not permitted during laboratory sessions (but if it is not causing any problems, then there is no need to enforce this).
Very rarely one of the authors has had to resort to drastic measures in order to force a student to comply with requests to leave, such as rebooting the machine they are using! This hasn't happened for a few years (perhaps his reputation precedes him nowadays!) but such a method of demonstrating your authority is still available as a last resort.
You should also keep an eye out for students who lock their screen and go away
for long periods. Our current policy is that students should take a break from
using a computer of approximately 10 minutes every hour, for obvious health
and safety reasons. Most do not in fact do this, and currently we do not force
them to. However, anyone who has been away for longer than this, or who is
using a lock screen that does not display the elapsed time and the user name,
should be logged out by you. On Linux you can do this by typing
<Control> <Alt> <Backspace> to kill the running X server: there is no
need to reboot the machine. Very bad cases should be reported to
John Latham
(email: jtl@cs.man.ac.uk), who will email the student and his/her tutor.
Notwithstanding all the above, most students behave very reasonably, and you are unlikely to have problems.