As a child gets tired easily and loses concentration due to thalassamia. This has affected his focus and resulted him not being able to focus and thus resulted in not able to perform well in school's work
It certainly can and most likely would. The length of the school day is driven as it is by practicality of schedule, not necessarily by what length of time kids require to learn, so those with extra challenges interfering with their ability to learn new material, up to and including conditions and syndromes which cause a student to have difficulty concentrating, would make it more difficult for them to learn in general.
It does seem to me that this condition would affect a child's schoolwork. After all, if the symptoms of the problem include fatigue and irritability, the condition would surely affect a child at school.
We teachers all know that a tired student performs worse than one who is rested. We also know that a student's emotional state matters. If the student is easily irritated, he or she will have a low tolerance for things going wrong (making mistakes in class, etc). Both of these symptoms could easily make a child underperform in school.