For the iPad, one app I especially like is Noteability. You can pull in documents from other file formats (such as Word or PDF), and it is easy to use a stylus to hand-write whatever you want from there.
My regular go-to choice for course work is still the Google Apps (docs and sheets). The Google apps are far more robust on the iPad than when I first started this, so it's worth a new look if you haven't tried them recently. A big plus is the FREE price tag. I have all of my online students create a folder on Google Drive at the start of the semester and share that folder with me. Once the folder is shared, anything put into that folder is automatically shared with the same people. So students can submit work in any file format, I can open, grade, and give feedback (the commenting tool in the docs is nice). I can even create group project subfolders for collaborative assignments using the unique feature that files and folders can live in more than one place on Google Drive! So the group subfolder can be added to each group member's shared folder, and it will automatically inherit the sharing permissions.
George Alexander
Madison Area Technical College