When you go to onRecord > Settings > Grading > Calculation and click the pencil icon edit a Grade Average. Select Step 2 School & years whereyou can select which school year and levels to include.
In some cases, a year may not be listed.
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.