Hello. You did not inform which article this question refers to, which prevents it from receiving an exact answer, however, I will try to help you in the best possible way.
When a student eats properly, receiving appropriate energy from food, that student has optimal health. This allows the student to be more attentive, willing, more proactive within the school, without appearing sleepy or lacking energy. In this case, an author who wants to show how school meals have improved students' health, should show this type of improvement as evidence of his statement.