Primary grades ?

Hi~ what could be the best interpretation of the words "the primary grades" in this context?

All elementary school grades OR the lower grades of elementary schools?

Thanks for your help in advance~

The ability to exercise self-regulation is dependent on the development of the prefrontal cortex to the point that it can exert control over the limbic system (primitive brain). A key reason schools should consider promoting self-control is because it has been found to be the greatest predictor of educational success. Self-regulation is developed during childhood but can also be advanced through late adolescence and even into early adulthood; however, there are fewer obstacles when developed early in life. Helping students develop self-control during the primary grades changes the trajectory of their entire academic experience. The prefrontal cortex is developed in predictable environments, through positive social interactions and learning. Cheating is a shortcut produced by the inability of the prefrontal cortex to self-regulate.