Aww App is a super simple browser-based application for creating drawings. To get started just go to AwwApp.com and click on "start drawing." To invite people to collaborate on your drawing just send them the link assigned to your drawing board and they can join in the drawing fun.
If you would like to use Aww App on your classroom or school website and you have familiarity with editing the code of your site, you can install Aww App for free. The free version of Aww App when installed on your own site does not allow sharing. For $10/month you can enable sharing. Please note, AwwApp.com itself is completely free with sharing options. The pricing only applies if you want to run it on your own website.
Applications for Education
There is not a ton options for your drawing tools on Aww App, but there are enough that you can certainly illustrate your point(s). I actually like that there is a limited selection of drawing tools because it will force students to focus on illustrating and diagramming their points rather than worry about making them aesthetically pleasing.