Children as young as four are suffering from mental health problems such as panic attacks, anxiety and depression, teachers say.
Almost all of the 2,000 who responded to an NASUWT survey said they had come into contact with mentally ill pupils.
Members of the teaching union suggest schools are struggling to access enough support to deal with the issue.
The Department for Education said it was investing £1.4bn to ensure all children get the help they need.
The union is highlighting the problem at its annual conference in Manchester this weekend and it will also discuss school funding and the online world.