Many schools offer students the ability to take college classes and earn credit towards an undergraduate degree or complete their major's pre-requisites while in secondary school through Dual Enrollment

Moreover, many schools also allow students to get college credit through Advanced Placement (AP) and College-Level Examination Program (CLEP) exams for certain subjects

This site offers online resources and outlines a multitude of courses that could either be done through a dual enrollment program or as an undergraduate in college

Disclaimer: Since the structure and content of these courses listed depend and vary by institution or instructor, it is strongly recommended to use the resources listed here as supplementary or a review to what is taught in these classes and not as a replacement for the lectures or required readings