Why if my daughter is so smart she can't focus in class and simple stuff like sit next to somebody when the teacher ask her to?

Kids with ADHD are distractable and move around a lot, but the question is does she display the same behavior in other situations? If it is only happening at school then more than likely she is not being challenged enough or on the flip side maybe she is struggling because of a learning disorder. I would have her evaluated by her doctor to see what could be causing this behavior.

My son had the same problem he was bored they said he had add but we stopped medicating him and started taking him to places where his mind could be quenched. Maybe she is bored and needs more.

I agree, possibly ADD or maybe she is just bored. Talk to her teacher about it and find out if it is a continual thing or if it is only at certain times. Or maybe she just doesn't like to behave in class. Get her tested for ADD to rule it out and then go from there.

ask school if they think it might be add or adhd
and they they think possible
have her tested from school and outside professional
if child does have then ask for an IEP

Smart kids get bored in classes that don't challenge them.

Possible ADHD? Have her tested. A simple thing to try at home is to give her a small cup of coffee- if she isn't bouncing around like a squirrel, then it's possible she has it. GL

Attention Deficit Hyper Activity Disorder or ADD which is w/out the Hyper Activity
there are alot of websites that you could visit for more info

Boredom

She is probably too bored at class specially if she knows everything the teacher is explaining already