First of all, make sure your students are in your list on your 'Students' page. Once students have either viewed something you made or created their own story, click on 'view stats'. There are several types of stats you can access:

Teacher shares story with students
See how long students are active on the story (browsing, viewing videos, answering quizzes, commenting etc.) and how they answer the quiz questions. Quizzes on Sutori are more formative than summative but it will only take into account a students' first attempt at answering a quiz.

Students create stories
See how long students worked on their assignments, particularly useful for collaborative projects (stories with more than one editor) as you can see who put in the most time. You can also access an overview of the items which is useful if you had specific instructions.
Below you'll find a recap video.