A parent or teacher, who the children or student trusts are allowed to tel them to stay away from certain people who have bad character traits like haughtiness, anger management problems etc. so that they will not learn from their ways. This does not constitute lashon hara and is even a mitzva to do because just as we learn later on that one can tell someone about a suspected plot to try to cause them to lose money or to get hurt and now they can take precautions to stay safe all the more so to keep your children or students away from someone who can spiritually, psychologically, or physically hurt them, because of his character flaws is certainly a mitzvah to talk and take the proper precautions. One does not even have to abide by the five rules before speaking lashon hara, meaning that even if he just heard second hand he can still tell them as long as he explicitly says it in a way letting them know he just heard it second hand and he should be very clear about why he is telling it to them so that they won’t get mix messages about speaking lashon hara. Also it is permissible to exaggerate a little bit, or to embellish the severity of what he or she is doing wrong just to be sure your children or students will heed to what you are saying. But that does not mean you should lose hope on this bad guy because he can always repent and might not know the severity of his ill character. However proper precautions to stay away from him should be taken and shared with those who would trust what you are saying like your children and students, anyone else you cannot talk to and it will most likely be lashon hara since they won’t necessarily trust what you are saying and do something about it. Lashon hara is to scoff and make fun of others for your own gain or there loss but when spoken to protect others from any sort of harm, not only is it permissible but it’s a mitzvah to be spoken.