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
Get Creative with Google Drawings from the amazing Tony Vincent
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.
Video Tutorial for Students
Google Drawing Cheat Sheet for teachers and students