The free services offered will get you going as a classroom teacher. The premium services, which cost about $5/month, allow you to give kids coins they can spend in the virtual store to modify their avatar as well as give diagnostics to teachers.
Here's information from their website:
Motivate your child to read every night, encourage them to write for their audience of classmates from your phone, tablet, or computer.
More Than A Reading Log
In addition to bringing modern technology to classic paper reading logs, Whooo's Reading:
Students earn Wisdom Points (WP) by reading, writing good answers to critical thinking questions, and more. Then students use their WP at the Owl Shop to personalize their Owlvatars!