Schools have dictating who you have as friends for quite some time now. For example when I was in school and had a party I invited whomever I chose. Now if you have a party you have to invite the whole class.(which i think is bullshit) Same for stuff like valentines cards, you have to bring them for the whole class.
I do think the idea of prohibiting a "best friend" is ridiculous. All children group together with the kids they feel comfortable with. Unless its a group of bullies I think the school system should keep out of it.
Learning how to move around and interact with uncomfortable social groups is a part of life. The sooner kids learn to navigate the path of introductions, getting along, making new friends and how to interact with other groups "not your own" the better off we will all be.