The "Perform Access control at program start" option makes the Master application check whether the teacher is allowed to access computers at all. It's an additional check for convenience (which can lead to false positives, that's why you should try to disable it for the time being) but not strictly necessary as the actual access control is performed on client computers so that only authorized users will be granted access. If the option is disabled, anybody can start the Master but still will only be able to access clients he's really allowed to access.