For the problem of using calculus in high school, calculus itself is actually a process of accumulation and summation, which is the same as the summation of series you studied, except that discrete variables are transformed into continuous variables, so the nature is the same.
In fact, to be honest, a large part of high school mathematics belongs to cultivating thinking ability and standardizing your problem-solving methods. Real mathematics is actually based on calculus, and calculus is an important method to solve practical problems in many fields.