As far as I can tell, there is no limit to the amount of students or classes that you can have. There also did not seem to be a username availability check. Another nice feature about this page is that there is a link to print log-in cards. You can have a set of cards for the classroom and send the cards home with the students. Below is a picture of what these cards look like:
As you can see this little card is very kid-friendly and simple to understand and use. Yay! The student page that it asks you to visit isn't much different from the general page except that it pops up with a student log-in. Once on the site, there is a mail button where if you allow the students to communicate, they can write notes to the class. There is also an inventory button that enables the students to see their saved projects and projects done by members in their class.