I see you got the answer to your question already. If you purchase the $60 account for the year, then you can set up student accounts easily. If your like our schools last year they either had the students use their e-mail addresses, not sure at 5th grade if you'd have that, so maybe 'make up e-mails' and sign up for an account where they get to create 3 VT's.
IF, however, you can watch them and monitor what they do I know quite a few people make 'Identities' under the teacher account, then sign in as yourself and have them switch to their identity when they want to comment.
Sounds like you already knew this, but wanted to make sure. Have a great start to the school year and welcome to our VT Ning group.