Mental health difficulties, in the main, are a hidden disability . It is unlikely that you would be able to tell anything about a person’s mental health when you first meet them.
For a variety of reasons, many students with mental health difficulties do not disclose their disability to their HE institution or to their tutors.
Some students with mental health difficulties experience periods of disturbance and distress and their altered behaviour may be confusing to tutors and fellow students. More rarely, behaviour can be alarming, often because of an ignorance of how to respond to people with mental health difficulties.
Violence is very rare in connection with mental health difficulties - there is a far greater connection between violence and alcohol use.