Children still in kindergarten or even younger form cliques and intentionally exclude others, say psychologists and educators who are increasingly noticing the behavior and taking steps to curb it.

Special programs are popping up in elementary schools to teach empathy as a means of stemming relational aggression, a psychological term to describe using the threat of removing friendship as a tactical weapon. Children also are being guided in ways to stand up for themselves, and to help others, in instances of social exclusion....