Here are a couple of suggestions for leaving feedback for students (particularly useful for when marking remotely). The owner of a library (and any co-teachers) can edit any of the books on the bookshelf, so you can just open the student books to leave feedback.
Dr. Monica Burns believes feedback needs to be timely, relevant, actionable and inviting. Watch her webinar for more insight.
Some teachers like to give text feedback using text. You could use a shape for this, and simply type directly into the shape. Students can read the feedback and then delete.
Make your feedback even more inviting by recording audio to explain further instructions or corrections. Make sure the audio hotspot is easy to notice as the student looks through the book (perhaps layer a shape behind it to make it stand out?). You might also consider adding a transcript of your audio for certain students.
You could even use the camera to record video for this purpose!
Feedback Sticker Pack
If you have a premium subscription you can access our Feedback Sticker Pack add-on from the Book Creator App Store. These stickers will help you (or your students) give encouraging and fun feedback with ease!
Use the highlighter pen
If you need to highlight particular text on the page, use the highlighter pen in the Pen Tool. How about a system such as pink highlighting for great work ("tickled pink") and green highlighting for errors ("green for growth")?
For younger students, why not set up a system for grading with emojis? This can be a fun and encouraging way to motivate students.
Collaboration really comes into its own when you have students commenting and offering constructive feedback on each other's work. Make sure you have turned on the ability for students to see each other's books, and turn on collaboration in the Library Settings.
Also make sure students know how to turn on the setting so they can see each other's books, not just their own. Use the dropdown underneath the library title and switch the toggle to See everyone's books.