I think the teacher marked the problem this way because the state exams under No Child Left Behind have been written to enforce a particular interpretation of 5 x 3. The student would probably be marked wrong by the anonymous scorer on the state exams. So it’s an act of self-protection.
If a student doesn’t know that 5 x 3 and 3 x 5 mean something different, how will the student be amazed when he/she learns that they are actually the same thing? (Written partly in jest, but not 100%)