By default teachers can only view the lesson notes from their own lessons. However, in the scenario that a teacher is substituting and needs to view notes from lessons the student has had with a different teacher, they can go to the student's profile and see the "Recent Lessons" section to view the student's 10 most recent lessons. By clicking on the lesson title in this section they can view the lesson details including any notes from these lessons.
If the tutor hasn't been assigned as a default teacher to the student (in which case the student won't appear in their Assigned Students table), then on the calendar the substitute teacher can click the lesson they have been scheduled with the student and click the "View" option in the pop-up. In the lesson details that are displayed, they can click the student's name which links to their profile.
If you would like to limit the "Recent Lessons" and "Upcoming Lessons" sections displayed on a student profile in teacher accounts to only lessons taught by the teacher viewing the profile, you can follow the steps in this article: Limit Teacher View of Student Recent & Upcoming Lessons to Only Their Own.