What is a reasonable expectation for privacy of exam and homework grades?
The answers in the comments are correct.  FERPA, the Family Educational Rights and Privacy Act, makes "education records," which include grades, private and protected by law in the United States.  Students can reasonably expect their homework and exam grades to be held private from other students.  (Students acting as TAs can necessarily see such grades, but are bound by FERPA to keep them private.)  A search for FERPA will tell you more than you could ever want to know.