A certain achievement in math is a common requirement, and the math courses that MBA programs require are split between statistics and calculus. For example, while Auburn University and Montclair State University both require college-level calculus, Empire State College and the University of California Santa Cruz require statistics. With some preparation, it is possible to take both calculus and statistics at the undergraduate level, ensuring that any math requirements are taken care of before deciding on what business school you want to attend.
A requirement found among some business schools is the completion of one year of economics--both microeconomics and macroeconomics. While Auburn University does maintain both as prerequisites, Empire State College does not require either. However, even though this is not a universal requirement, many schools will appreciate applications from students who have completed them because they serve as good preparation for the MBA curriculum.
Although the MBA is a business degree and MBA students learn advanced business terms and methodologies, most graduate business programs do not necessarily mandate that applicants take specific business classes prior to applying, particularly if they have some experience in the business world. Those that do have such prerequisites typically require some combination of accounting, finance, management and marketing, which, even if not required, are suggested since they are also good preparatory classes for any MBA curriculum.
There are no hard requirements for classes you must take in high school to be eligible for enrollment in an MBA program after college because all requirements are at the college level. However, you must make sure you are in position to take those classes later. Additionally, taking related classes in high school will help your application to a graduate business program by showing your early commitment to getting your MBA. One place to get started is in math, ensuring that you will advance enough in your mathematical understanding to take calculus in college. Economics and accounting courses are also often offered in high school, which will help you when you take those courses at the college level later. Additionally, high school students would be well-served by taking Advanced Placement (AP) courses which are offered in calculus and micro- and macroeconomics that can earn you college credit.