From a former U.S. student: School is usually referring to grade school, so a school year (which is typically fixed in time; e.g. September through June)is best used when referring to elementary or high school. An academic year better fits with university studies, as in the 2018-2019 academic year. Students at university often study in summer and/or take semester breaks, and still finish the required studies in an academic year. I hope this helps.