Well, this is her first step to independence. She will have to learn to negotiate- or to be happy playing alone. Not something that you can do much about- she has to do her own growing up.
All you can do is suggest strategies: negotiation (if we play your game at breaktime, we can play mine at lunchtime), finding other friends etc.
But mostly what is required of us as parents is to lend a sympathetic, but non-interfering ear when a child wants to rant about their rotten day. Provide her with her own little personal mumsnet
Some kids are happy to be play alone- if she turns out to be one of them, it's not for you to wish her different, that might just be how she is.
If you think she is being deliberately left out, then speak to the teacher. Maybe there is something like a buddy corner where lonely children can find friends.
But as teacher has said she is sociable, I would just let her rant but not take it to heart. Point out the need to compromise once in a while, but don't agonise if you can't turn her into a totally different person overnight.