I know I want to be a counselor, what is a good class to take in highschool?
I agree with James's answer of Psychology and Sociology if your high school offers them.
I'd also suggest getting to know, or even shadowing, your high school counselor to see what they do and what issues they run into on a daily basis.
Lastly, I'd say to involve yourself with as many extracurriculars and after-school clubs as possible to expose yourself to a diverse group of people and scenarios. This will help you be more empathetic as a future counselor.
Brandon, thanks for posting this question; it's definitely worth exploring as you consider the next season of life, specifically your career ideas. In terms of the courses that will best prepare you for work as a counselor, I certainly agree with the previous answers. Psychology and Sociology, if it's offered at your school, are both good course options. I'd like to offer a third course suggestion - English or Communications. Understanding human behavior is one thing. Learning how to best communicate - both orally and in writing - with those seeking your insight, guidance and encouragement will be a very important tool to carry with you at all times.
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If your high school offers these classes, take Psychology and sociology. Psychology especially will give you the basic knowledge of how humans think and rationalize. These courses will also be required as part of your counseling degree in college.