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 Drawings Tutorial for Students (14 min)
Google Drawing Cheat Sheet for teachers and students