Who can see a student's grades?
Officially and in general, no.

The teacher of a class should in general not share grades with his colleagues. Informally, however, there might be some communication of individual students' performance in ways that don't violate confidentiality: "she did very well on my class"; "he struggled on the exams, but did a very good job on the final project." Typically, this would be in the context of a colleague inquiring about hiring a student as a student worker or as a graduate student. Aggregate information could also be shared without harm to enable better matching of teaching to student needs and abilities.

That said, there are exceptions to this rule. For instance, your academic or research advisor will typically have access to all of your grades. Similarly, an evaluation panel—such as one that convenes for a graduate student's qualifying exams—will probably have access to all of the student's academic record.

But a random professor generally will not see a student's grades in all courses. This is by design and a logical move.