We have a small church, and many times the preschool through sixth graders are in a group together. Number the children, and ask them to remember their number. Or, you can just use names. Everyone stands in a huddle, and IT throws the ball straight up in the air and calls out a number. The child with that number has to stay and catch the ball while the others run.
Play continues as long as interest holds.
We have enough problems finding teachers without having to separate the boys from the girls. I have been teaching for 23 years and my classes have always been co-ed. I love watching them interact especially in games of cooperation. Sometimes during sings we have boys against the girls. In 1985 my first grade son learned bible verses that coincided with the alphabet.