Scratch Jr. is available on student Chromebooks through the Google Play store. Open the store with the triangle icon on the bottom of the Chromebook home page and search for Scratch Jr.
The LEARN section of the site provides teachers with a pdf guide to help students get started using the tools in Scratch Jr.
Teachers can use ready made activities in the TEACH section of the site as starter activities for students learning Scratch Jr.
Click HERE to access a brief slide deck to introduce you and your students to Scratch Jr. including how to access it on a Chromebook.