When you go to onRecord > Settings > Grading > Grade Averages and edit a Grade Average, you will see on the left hand side Grade Average Details. Underneath this area when you click on Grades you can select which school year grades you want to include in the Grade Average.
If a school year is not listed, go to Core > School > Years and Terms and adjust the school year so that Publish is set to Yes.
If you are trying to add a future year, you need to also set the Current Year to Yes on the future school year. Once it is added to the Grade Average setup, you can change the setting back to Yes on the actual current year.