Op-Ed article by Timothy P Shriver and Robert P Weissberg says promoting students' social and emotional skills--process by which children learn to recognize and manage emotions--plays critical role in improving their academic performance; describes some social and emotional learning programs; says recent studies vindicate what has long been common sense among many teachers and parents: that children who are given clear behavioral standards and social skills, allowing them to feel safe, valued, confident and challenged, will exhibit better school behavior and also learn more (M)