The curricula at most BS are very prescriptive, requiring four years of math, language, science, and English/writing (regardless of the the level at which you enter each stream), leaving very few slots available for electives which are mostly taken as a sixth/additional course. In our son’s four years at Choate, he had room in his schedule for three electives. Theoretically, electives can be take in any year (with permission, our son took a sixth course freshman year), but each year you will be taking a course in each of the four major disciplines. In the upper years, you will have some latitude in choosing among various offerings in each stream if that is what you mean by electives.
Boarding schools are not flexible on requirements, but there is some flexibility in how you meet those requirements.