First Grade Social Development
Boys are friends with boys and girls are friends with girls.
The two genders tend to separate into their own social groups in first grade. Girls form many friendships, but they can be fickle. A friend today is an enemy tomorrow. Boys have smaller groups of friends, but these friendships generally last longer.
What you should do: Accept this trend as normal, but don’t worry if your child's best friend is of the opposite sex.