Kids self esteem takes a beating in middle school, maybe worse than anywhere else in life. Middle school is when cliques form, puberty starts (for most), and kids are coming into their bodies.
Let's start with cliques. Kids are forming groups based on, who knows these days? When I was in school it was pretty much looks. All the cute girls and guys were popular. These days, I think it's the loud obnoxious ones. The ones who make you pay attention. When my oldest daughter started middle school, there were two HUGE cliques and then everyone else. The friendships she had wavered that year, you know how fickle middle school girls can be.
Now my youngest is in sixth grade, and I've already seen her friends choose sides, leave people out, gossip. It starts all over.
Luckily, the oldest girl has found her way, being a friend to everyone, and having her core of good friends. My youngest daughter has also naturally adopted the "friend to all" attitude, and I think that will help them both in the long run.
Then there's puberty. Nothing damages a girl's (and probably a boy's) self esteem more than a huge zit right in the middle of their nose. Lucky for my kids, I have naturally oily skin (being Italian and all) and keep a vast array of skin cleansers in the house, clearisil, nutragena, proactive (although I have no idea how to spell these things obviously). Unfortunately, I see so many kids with bad skin, nice kids, and you wonder, why don't the parents help them out with this? Zits (well most, there are those big mothers that pop up every once in a while that just have a mind of their own) can be helped with good hygiene.
Then there's weight. My daughter, at 105 pounds complains. She's not fat. At all. My younger daughter on a daily basis asks me if she's chubby. I try and stress to my daughters that they are not fat, that they do not have to worry about it yet. If they eat right and stay fit, they'll never have to worry about it. I try and eat right and exercise to set a good example. I don't buy sweets, only on occasion, I don't by sugary cereals, pop, or candy regularly. I buy them ice cream about once a month, chocolate milk about once every two months. Being overweight can completely damage a kids self esteem, and I don't want my kids to go through what I went through. (and not having these sugary sweets can also cut down on the acne)
As a parent, I do my best to help my girls feel good about themselves, teach them how to take care of themselves and how to be a good friend. The rest is up to them.