Set your default academic system, add terms per grade, and indicate the Grade default per level. You also have the option of customizing what you call a particular grade. eg. If at your school, grade 12 is termed as year 13.
Getting there:
Go to your Account Configurations and scroll down to the section on Grade Default.
After making changes, don't forget to click 'Save'.
Note: Only Administrators have access to this page. Any changes made in Account Configure apply to all users associated with your account.