This week ReadWorks announced a new offline mode for students. This allows students to download articles and assignments while connected to Wi-Fi at school and then use those materials on their laptops, phones, or tablets at places where they don't have Internet access. Here's the official announcement and tutorial that ReadWorks published earlier this week.
It's important to note that the offline mode in ReadWorks doesn't support the audio or paired videos features that are available in the online mode in ReadWorks.