You asked

My daughter wasn't invited to a classmate's birthday party and everyone else was. What should I do?

It’s not a nice feeling being left out but it’s unfortunately part of growing up.  Your daughter isn’t going to be invited to every party. There could be many reasons that your child wasn’t invited:
Not all parents have the energy or money to invite the whole class. It’s common for guest lists to become smaller as children grow older. It’s also true to say that not all children will get along and the birthday girl may have decided your daughter just doesn’t make the list. In both cases, there isn’t much you can do about it. It’s the party thrower’s prerogative to decide who makes the cut.
Every parent will want her child to be liked, but this isn’t possible and it just isn’t going to happen all the time! As hard as it may be, swallow your anger and help your child work through her feelings of rejection. Remind her of all the friends she has and all the people (including you!) who love her. Remind her of all the parties that she has been invited to and even though she isn’t going to this party, there have been lots that she was invited to and even more to come.
When the day of the party arrives, let her have a friend who isn’t in the class come over or bring them to a movie. She will be less likely to spend the day thinking about what she’s missing out on and focus on celebrating the wonderful friends (and parents) she does have. 

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