To add Aircraft Rates go to Manage > Aircraft > Select the desired aircraft > Rates Setup tab > +New Rate.
You have the option for Allowed and Default. You will select which Groups apply here. To add a new group go to Manage > Groups > +New Group. Click here for more information on Groups.
Make sure to apply the Instructors Group to Allowed if your instructors are checking-in users, that way they will be able to select the appropriate rates per user.