Annotating with Skitch
Skitch was one of the first apps I learned about after I started using my iPad. It's a free annotation app that uses text, shapes and drawing tools. Skitch gives you the option to annotate on photos from your photo album, a picture you take with the camera, a map, website or PDF file. You can even start with a blank page and put anything you want on it.
Use tools like text boxes, shapes, free hand draw tool and sticker labels to develop your graphic.
Still need help deciding how to use Skitch? I did a quick search for "using Skitch in the classroom" and here's what Google found for me - https://goo.gl/dJOjLA.
I'm sure there will be no shortage of ideas for your classroom once you get started!