Understanding the differences between primary and secondary Tumblr blogs is helpful:

• Primary Tumblr’s are best for your public, “this is me!” profile.  You can comment on other blogs, but you can’t invite coauthors or use password protection.
• Secondary Tumblr’s are good for classroom collaboration – you can invite coauthors, and then password protect your work.  This would be a good way to introduce students to Tumblr without the public spotlight.