Explain Everything is not free but it comes at a very modest price. It is a cut above the free Educreations which is an app I really like for its flexibility. Explain Everything is iDevice compatible and works best with Apple devices. I have the app on my Macbook and my iPad. It is something which can be used for presentations, for teaching and recording that as you go along. You can use a stylus/finger or typed next. I made a montage in Fotor, imported it and then added what I wanted. you can save the work as video, image and to various places like Dropbox (my preference) or you can email the work to yourself or someone else. It really is very flexible and worth investigating for you and your students. Excuse my appalling drawing skills.