First. Drugs and alcohol use. You might notice them irresponsible behavior. Sometimes they talk about suicidal like talking about death or dying.
Second. Recent changes in sleeping or not sleeping or not feeling well or eating habits. Personalities. Difficulty making friends. Loneliness. Listlessness. Problem at school, including class cutting, for being absent and also drop in class/grades. This may cause a long depression and it's precedes a suicidal attempt.