The most likely cause of a code error is that the code has expired. (You can see this by checking the expiration date on the email that originally provided the code.) Your classroom code works like a coupon - you can use it to set up your/your students' Plus accounts, but it won't work after the expiration date.
To apply for a new code, fill out the application here:
If you are sure that the expiration date for your code hasn't passed, double-check that you're entering the code exactly. For example, make sure you're not switching up your Os and zeros, or your Ls and ones.