Can you tell me what was wrong with my smile?
First of all, I like your hair style. It looks very cute and easy to manage.
To answer your question, it would be very difficult for an average person to describe what is wrong with your smile. Some perceptive people might say that it looks a little forced, which is often seen as fake to most people. In fact, actors use a "forced smile" look to characterize a "fake" smile, when they need to show they are patronizing someone or showing distrust.
The main thing that strikes me about your smile in that photograph (it's not really your smile, I'm talking about - it's the way this particular photograph makes you look! I hope you can understand the difference.) is that it does not "agree" with your eyes. Your eyes and your smile don't "match."
If you were happy enough to smile, the place a true smile is first born is in the eyes. The beginning of a smile is always in the eyes and then the mouth. If you were really smiling that "big" your eyes would really show it more. A lot more! Your expression would cover your entire face evenly and maybe even, with a smile that big, close down your eyes a little.
Now, to let you off the hook a for a moment, most school pictures show an expression just about like yours in this photograph and most of the time, moms love it - "Hey, she's smiling AND her eyes are open! What a great picture!" - when the reality is that it does not really look like you. Not that I know what you look like, but I know a bit about facial expressions after spending many years doing portrait work and studying to understand the differences between a portrait that will sell (better, yet, make mom cry, because the more Mom cries, the more money Dad pulls out of his wallet) and one that looks like what every one has twelve of - one from each year in school - already hanging on Mom's wall.
This is not a criticism of you, but if you were honestly asking for some pointers on smiling more, don't look at your mouth in the mirror or in photographs and start looking more closely at your eyes.
