Something to note: If using a buddy cord... the TX tuned to the aircraft (held by the instructor) can have expo... and the setting is NOT passed to the buddy box.
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The student should not be using expo or dual rates or any other crutch to change the aircraft response. You want the trainer to respond to the controls in a way that show when the student makes an error.

The primary use of Expo and dual rates is to aid in handling an aircraft that is hard to fly. Secondary is to improve precision. Unfortunately, these capabilities on modern radios are being used as a crutch supporting poor flying skills.