BUG ALERT! Back in 2019, "Attendance" button/link was added to group page schedules. However, that button is not linked to Group Page Access settings. Even if teachers don't have access to their rosters, they can still see their students by going to Schedule, and clicking on "Attendance". The only solution is to uncheck "Take Attendance" on every single course that has attendance, and then check it again when teachers can have access to their rosters. THERE'S NO BULK UPDATE METHOD.