As a math tutor of students from elementary school through college, I would say that Calculus should not be made mandatory for a few simple reasons. First, many students haven’t had the required prerequisite courses (2 years of algebra, geometry and trigonometry) to be prepared for a high school level calculus course. To obtain those prerequisistes Algebra should be taken no later then 8th grade, with Geometry taken in 9th grade, Algebra 2 in 10th, then Trigonometry or a pre calculus in 11th. Some students take Algebra in 7th grade which enables them to take a first year Calculus course in 11th and a second year course in 12th grade. Those students not on the course path to prepare themselves for college will never be ready for a calculus course, which is fine because many of those are in vocational tech where a course such as calculus will be of absolutely no use to them in their careers as an auto mechanic, print shop worker, electrician, plumber, carpenter, etc.
Many students who are very intelligent in mathematics may wish to take a calculus course even if they aren’t going to enter a mathematical or science based curriculm in college becuase of the wide range of thought processes involved in understanding the concepts of calculus (such as the intergal, derivative, limit, volume of three dimensional figures rotating about the x or y axis, etc). Many mathematical results can be proved and such thought processes involved in proofs are beneficial in other fields such as psychology or philosophy.
I suggest calculus to the students I work with that want to study mathematics, statistics, engineering, computer science, physics or chemistry at the university level. Every one of those cirriculms will require courses in Calculus and students who do exceptionally well in high school Calculus may take an exam and place out of college Calculus. By placing out, the student earns credit for the college Calculus course and can move to the next course in their college curriculm.
To force students to take courses which they simply will not use or never will be prepared to take makes no practical sense. Some students just do not have the mathematical skills or ability to take the course at the high school level. I believe to teach students as much as they are capable of learning, as long as it benefits them in the long run outside of school in the real world.