Aviary is one of my favorite web apps. I use the Aviary extension in Google Chrome to take and edit screen shots and I use some of their other tools to edit and modify graphics.
Aviary Education is currently in Beta and is offering free accounts for educators. The site allows educators to create private student accounts, manage assignments, and use the image editor, vector editor, audio editor, and music creator with their students. The content is promised to be 100% school safe.
This is a great tool for all educators to use, especially graphics, art, and music.