When I was a med student you didn't choose which classes or which professor. There were 4 classrooms with all of their subjects organized to fit. In some semesters the classes classroom A and C saw in 7th semester were what classroom B and D saw in 8th semester. Sometimes people of different semesters of the same school year shared classes.
You had a certain degree of liberty in choosing which of the 4 clasrooms you were interested in (schedules and professors didn't change much, so if you wanted "x" teacher for ORL you requested the classrooms where that teacher taught. However, it was more of a thing of politics and a lot of luck. Only once did I get the desired classroom and it was only to knock out the hardest subjects of my 4th year irst and have the easy subjects in my 8th semester.
They changed things now so to some degree you choose filler subjects in whatever order you choose. You can temporairly drop out of the career and have a maximum of 3 semesters to come back to continue the next semester and can do this until internship where you're forced to complete the 12 months in one blow or you're screwed. If you owe any requirement of any sort, you can't graduate. You lose automatically a semester and have those 6 months to try whatever procedure you missed again.
Don't know how different things are in the US though.