Cindy,
I wasn't positive but it does look like the mobile apps are available with the Free for Teachers version of Canvas. The URL for Free for Teachers is canvas.instructure.com; type that into the login screen to authenticate using your Free for Teachers credentials. Instructions are in How do I log in to the Teacher app on my iOS device?
More specific to your question, Grading features are only going to be available to you by using the Canvas Teacher app so that is what I'd suggest. Sometimes it's also nice to have the Student app on your phone to see the course more similar to how a student does, but most of the tools you'll want are in the Teacher App.
Rick
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