Your daughter being tired at this age can be due to puberty and the changes taking place not only physically but mentally as well (children of this age should be getting as much sleep as new born babies).
All children love sugar - this is not indicative of anything unless she uses the sugar to give an energy boost. This is counterproductive as the body 'crashes' after high sugar intake and we feel just as tired, if not more so, than before.
Putting an incentive plan into place where she can gain something by bringing home (realistic) good marks may help. It may well help to have an assessment done to determine which grade she is currently on par with to determine the type of assistance that would benefit her most.
In terms of vitamins, encourage a healthy diet above all else. You can supplement with omegas to aid concentration. A B-vitamin complex can also assist with fatigue. Please consult your doctor prior to giving any supplements.
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