Can one student see another student's grades?

No. A student can only see his or her own grades. You, as the teacher, can see all the grades of all the students in your class, but, unless a student knows the user name and password of another student or comes to a computer where another student has not logged out, a student cannot see another student's grades.

If you want to see the grades the way a student sees his or her grades, go to "Grades" under Administration (left side of course page). Click on the "View" tab and select the "User Report" link. The right hand side of the page will have a drop-down menu where you can select any (or all) of the students in your course. This will give you a page that displays what the students see when they look at their grades.