This is right out of the 2017 AMCAS Instructions
Year in School
The guidelines below will help you determine your Year in School. AMCAS is not responsible for any delays in processing and/or incorrect GPA calculations that may result from incorrect Year in School assignments. If the Year in School designation greatly exceeds the ranges specified below, AMCAS may reassign statuses. Each undergraduate status should consist of approximately 24-35 semester hours. If you are enrolled as a full-time, continuous undergraduate at an institution, you should usually change your Year in School after every 2 semesters, 3-4 quarters, or 2-3 trimesters. If you have been enrolled part-time or have had interrupted attendance, you should use these ranges to determine your appropriate status for each term:
High School (HS) College-level coursework taken while in high school
Freshman (FR) 0-35 semester hours
Sophomore (SO) 36-65 semester hours
Junior (JR) 66-95 semester hours
Senior (SR) 96+ semester hours
If you have a significant amount of AP or college-level coursework credits prior to entering college, you may list your entire first college year as Freshman (FR) status. In doing so, you will most likely exceed the 0-35 credit hour guideline for freshman year, but your AMCAS GPAs will reflect your “true” first year in college; in these cases AMCAS will not change the academic status. However, if you would rather base your academic statuses based only on credit hours, your application will be accepted and verified as such. The following rules will help you determine the appropriate AMCAS Year in School for all of your coursework (including foreign coursework): Do not assign more than one status to an academic term. Assign High School (HS) status to college-level courses taken prior to the high school graduation date you entered in the Schools Attended section, regardless of the physical location of the college-level course. This does not include AP, Exempt, or CLEP credit. Assign Freshman (FR) status to Advanced Placement, Exempt, or CLEP credit awarded when you first entered college. Once your undergraduate Year in School (FR, SO, JR, SR) has advanced, do not assign the previous status, continue to enter your coursework completed from that point.