Yes. Both the Javascript Blocks Editor and Python Editor can be used when your computer is offline providing they have been fully loaded in the browser beforehand.

JavaScript Blocks Editor

Providing you don't clear your browser cache when you close the browser, you will be able to return to the previous state of the editor, which could be useful on managed workstations/terminals. However, you won't be able to browse your projects, examples or tutorials when using the editor offline. 

Python Editor

The Python Editor will reset itself when reloaded and you will also not be able to browse help when working offline.

WIndows 10

The Windows 10 app beta also works offline, but the documentation will only load when online. Some schools have distributed the documentaiton on memory sticks/desktop folders.