I think Nicole's advice is very good for classes while in High School. I'd add that if you have any clubs that volunteer at your local elementary schools, take advantage of that! Take every opportunity to work with children to better understand them and what engages them. When/if you go to college - you'll have lots of courses to choose from to help you understand and work with children.
I found this free class you might want to take (only 8 hours of time): https://www.open.edu/openlearn/education-development/childhood-youth/introduction-child-psychology/content-section-0?active-tab=description-tab
Introduction to child psychology
After studying this course, you should be able to:
* understand the discipline of child psychology as an area of study
* understand how theories try to explain children’s development
* discuss important questions that are central to child psychology
* understand how child psychologists work in different applied settings.
I've never taken this course, but it might be worth checking out.
If you have access to courses at a Community College or University, there is typically one or more offerings actually titled Child Development. These courses will focus predominantly on the development of children within their minds and physical abilities as well as emotional growth over time. There are other courses like Children's Literature that might focus more specifically on a child developing literacy and reading skills. There are often courses you can take around teaching pre-school or daycare age children that will also delve into developing math, problem-solving, and socio-emotional skills.
If you do not have access to courses through your high school or a college/university, there are often numerous online courses or webinars that are available to paretns around Child Development as well.