Addiction, brain damage, poor choices made under the influence that result in injury or death, domestic violence, deterioration of entire families if getting high becomes chronic, rejection of employment or unable to maintain employment, if you are talking about illegal substances - arrest, incarceration, police records that will follow you everywhere (unless you have the money to hire a lawyer able to expunge drug arrest records - good luck with that one), diminished cognition, memory impairment, cardiac, liver, pancreas and other organic system damage, psychological changes - paranoia, suicidal thoughts, rage, panic, loss of appetite or its opposite, perceptual changes that may be terrifying, dangerous, or lethal, isolation, depression, institutionalization, skin / teeth disfigurement, loss of basic bodily functions and bodily functions control, horrifying withdrawal ... I think you get the point - keep in mind that all of the people encouraging you to get "high" may not react to the substance as you do - some people can occasionally get "high" and walk away from the drug - others will become addicted - we are all wired (genetics) differently, and grow up with our individual life events that may lead to chronic use and addiction. Why take the chance?
One of the dangers of getting high is the increased likelihood of getting into arguments and fights, which can be dangerous. Secondly, you are more likely to be involved in an accident when you are high. It also makes it hard for students to concentrate in class, thereby contributing to low grades.