The best method for managing a Single Instructor License is to have students create their own, free accounts, first, and then send them an invitation to join the instructor license. With a single instructor license, the instructor is given 50 credits in order to create 50 new student accounts within that license. Once all the credits are used up, however, additional credits can be purchased. One benefit of inviting students into your license (or creating the accounts with the credits) is that students then are promoted to a 'Basic Account' which enables them to create a maximum of 50 of their own VoiceThreads as opposed to just commenting or editing ones that you've created.
If you don't anticipate students creating any of their own VoiceThreads or more than 5 of their own VoiceThreads, consider just inviting their free account to join a group instead of inviting them to join your license. Instead of creating the accounts for students, you can send students a group 'sign-up' link. From here they can either sign up with an existing account or create one while also accepting your invitation to join the group.