The same way they have done over the years ... The children know that teachers could come in at any point if they had to, and also an element of trust.
Relying on a teacher being present is like relying on technology for certain things. It masks a problem and doesn't deal with it. To beat bullying you make it easy for the victim to report it, knowing that the bully will be dealt with and the culture of the school responds heavily to that sort of activity.