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There could be many reasons of her headache but most common at this age could be tension headaches
or eye strain
.First of all confirm if it occurs only while she studies and not during playing or watching TV, if yes then you need to counsel her.also ask her if she is stressed or anxious about studies.if not then get her eyes tested.Ensure that she has adequate sleep, a balanced diet and plenty of water.You can visit a pediatrician
if the problem persists.