Google Drawings is a free, web-based diagramming tool and a G Suite for Education core service. It's very versatile as well as being appropriate for all ages. Here are some resources to help you get started.
Do More with Google Drawings - Tutorials, Templates, and More
Activities with Google Drawings from the amazing Tony Vincent
Ditch that textbook: The Google Drawings Manifesto for Teachers from Matt Miller
Google Drawings Resources from the amazing Eric Curts. Tons of topic-based examples and templates on how to make graphic organizers, interactive images, posters, etc.
Tutorials and Guides:
Video: Google Drawings Tutorial for Students (14 min)
Step-by-Step: Beginner's Guide to Google Drawings
Cheat Sheet: Google Drawing Cheat Sheet for teachers and students
Edublogger: A Guide To Google Drawings
Blog: What is Google Drawings?
Google Support page: Create, insert & edit drawings
Google Applied Digital Skills: