I would say to my kids that It is ok, you do not have to know who is your best friend is. You will know someday. Also you do not have to have a best friend all the time. You can help your friends, but do not change yourself just to be with someone. Do not rely on someone, always have your own opinion. And if you have any question or any trouble, always always talk to your mother.
School is temporary place in your life. Every group you join, you will meet a lot of people you get along or not get along with. In that many process in your life, you will find good friends.