Dd's infant school is excellent at making sure all children are included at playtimes.
They have a friendship stop, and every week two Year 1 or 2 children are chosen (they have to apply in writing) to be Playground Friends and make sure that no-one is left alone. They wear bright yellow caps and big badges so everyone knows who they are, and children can either approach them directly or wait at the friendship stop for them to come over. Other children tend to go over as well, if they see someone waiting.
The school has a fabulous Children's Mentor who is a counsellor, and who has been trained in mediation with children - if she becomes aware that a child is being left out at playtimes, or if there's any bullying or teasing going on, she has a number of techniques that she can put into play. Sometimes it's as simple as finding someone for the child to play with, or giving them some kind of responsibility to boost their confidence (calling in different classes for lunch, for instance), sometimes she sets up friendship circles or team games for the children involved. She also runs groups for children who have problems in the playground, usually because they aren't very confident, which help to teach the child the kind of social skills which are useful in playground situations.
During lunchtime play, each class has its own play leader who organises activities for their class of children. They organise team games and encourage children who don't have anyone to play with to join in, and they are expected to keep an eye out to make sure none of their children are being picked on. This doesn't always work work because the class doesn't always want to play together and in a playground of 270 children, one member of staff can't always keep an eye on 30 individuals, but the children know that can always find a play leader or the Mentor if they need to.