I also had a weird credit situation (crapton of AP credits, took a semester off, 2 semesters of 20+ units), and called up AMCAS to see how I should go about classifying Year in School. They just said go by semester - 1st/2nd semester = Freshman, 3rd/4th = Sophomore, etc. They said even though my credits definitely didn't fit their standard 27-35 credits/year rule, they wouldn't reclassify my years as long as I followed the semester rule for classification. You can call them up too, but the lady I talked to seemed surprised that I was worried about things being reclassified because of my credit situation. She said as long as you go by the semester rule, and don't assign classes in the same semester to different Years in School, AMCAS won't touch it. Often, if you look at your transcript, it will list your year in school at the top of each semester. As long as that agrees with what you enter on your AMCAS, you'll be fine. (For example, I took Fall semester my Junior year off. My 5th and 6th semesters on my transcript say "Junior Year," so that's what I classified them on my AMCAS, even though the 5th semester was Spring and the 6th semester was Fall).