How I Use It
It's easy to install if you have admin rights. Scratch provides a guide and some cards with tips. I make students go through both of them before starting on a project. My first project is usually a predator-prey game or a maze. Maze rubric: https://www.dropbox.com/s/4zvi8a6zxw33rvv/Maze%20Rubric.pdf
I have used Scratch for a couple of years in a junior high programming class. The amount of programming knowledge required to successfully use Scratch is near zero. It is a great tool for the junior high age. It is listed as K-12, but I'm not sure how much of it could be used for programming with the youngest of audiences. Scratch is a wonderful introduction to programming, however once you complete a couple of projects most users hit a ceiling compared to other programming languages.
This is often students' first open-ended opportunity where they really have to fight through "bugs" and project management on their own. Some of my students who are not good at math still really excel at programming. Scratch (and programming in general) offers a creative outlet to students in districts without an art program.