I had18 students. I only used the administrator log in and put all students under one portal. Unless they have changed the number of students allowed, this worked last year. Good luck.
The 21classes site also has a message board function which posts in real time. This may help. You can also disable the feature that requires you to approve posts. Good luck with this!
I used the free version of this blog site 21classes with my 8th graders last year. The full control,no email required, only memebrs of my class could view and as administrator I had to approve anything before it was posted. Takes a bit to get it all…
I too am wondering what sites you are thinking of that require sign up. Many sites that are "educational type" do not have a sign up requirment with email. What types of sites are you looking for, this is 5th or 6th grade?
With your interest in 21st Century Schools, I recommend you take a look at Wiziq's virtual classroom and authorstream's power point presentation platform. Both are web based platforms, have a bunch of features and free basic service.
Here’s a Public Class on Technology Integration you might be interested in.