As you can see at http://info.scratch.mit.edu/Source_Code, the Scratch license seems to be MIT. Textually:

"The Scratch source code license allows you to distribute derivative works based on the Scratch source code for non-commercial uses subject to the following restrictions:

1. you cannot use the word "Scratch" to refer to derivative  works (except in the phrase "Based on Scratch from the MIT Media Laboratory"),
2. you cannot use the Scratch logo or the official (copyrighted) Scratch cat in derivative works,
3. you cannot implement the ability to upload projects to any MIT Scratch website (currently, http://scratch.mit.edu),
4. copies or derivative works must retain the Scratch copyright notice and license, and
5. you must make the source code for derivative works available.

This is just a summary; the full license is in the file "license.txt" included in the source code download package.

The purpose of these restrictions is to avoid confusion in the Scratch user community between the official version of Scratch (supported by the Lifelong Kindergarten group) and experimental variations. We are happy the share our source code, but we want to protect the Scratch brand and our user community."

You can find the license.txt file attached.