If my school only has two semesters in a school year, I should simply leave the "Third Semester" blank and use "First Semester/Trimester" and "Second Semester/Trimester" sections to list my courses?
just leave one blank if you only have two semesters. If you have quarters, maybe you could say yr-long or 3rd/4th qrtr or something like that for your core courses so you don't have to bother using add'l info space for this info?