If the coupon is expired, the student will be notified during the application of the coupon. They will not be able to add the coupon, and must pay for the test entirely on their own.
If the coupon has already been redeemed, the student will not be able to add the coupon, and the student must pay for the test entirely on their own.
There also may be a maxed out number of attempts for any given coupon code per student. This is usually 5 total. For example, a student takes the test and uses headphones for the first five attempts of the test. The coupon will then become invalid for the student to use, since they have already had multiple chances to follow the instructions properly.
If a coupon has already been redeemed in the account, but the student has not completed a test, there is no need to insert the coupon again. The student may simply follow the link that you have provided to them and start their test.