Does a communal breakfast constitute a violation of Islamic law after its prohibition by the Ministry of Education, and is it obligatory to denounce those who partake in it and leave the place if it is considered a reprehensible act?

The employee is a private hire, and their time is owned by the employer. It is not permissible for them to occupy their time with anything other than work, except with permission. Their violation of this is a breach of contract, based on the Almighty's saying: "O you who have believed, fulfill [all] contracts," and the Prophet's (PBUH) saying: "Muslims are bound by their conditions." As long as the employer has prohibited communal breakfast because it wastes time, it is not permissible for employees to do it, and it is permissible to rebuke them if they do. As for leaving the room, if it is your workplace, do not leave it. Otherwise, your leaving it depends on the expected benefit from doing so, and it might be a form of disapproval.