What page are the teachers viewing to see the last access?
What page are you viewing to see the last access?
You could be experiencing the issue that is here MDL-32275
On the "Participants" page in a course, it actually shows the last access for the Course. If you look at the "Enrolled Users" page it shows the last access for the entire site. As well as any logs an admin can see besides the "Participants" page.