Answer: There may be a correlation between a diet high in vegetables and better grades.
As with any scientific experiments, the results do not indicate a certainty, especially with only one study/experiment. In this experiment a correlation between the two factors is possible. However, more research is required in order to determine: that school children shoud try to eat more vegetables if they want to improve their grades.
Once a possible correlation/explanation is found, this result needs to be confirmed through more additional studies. This is the standard scientific procedure.