I had some bad classes in high school and college.
I don't know if Vector Calculus is the same as Differential Equations many others have mentioned here. I took VC in my freshman year of college, having placed out of Calc I and II due to good grades on AP exams in high school. I think I had the same professor someone else mentioned here. He was a Chinese man who spoke with a very heavy accent, making him tough to understand. Early on, I made the mistake of asking a question about something, and I got the same heavy-accent gobbledygook he said the first time. I kept my mouth shut the rest of the semester, as did everyone else. Somehow, I got a B in the class.
But that wasn't the worst class in my freshman year. My college (NYU) had expanded its liberal arts requirements that year, and the list of courses in the many required areas looked dreadful. I took a course called "Power and Politics in America," as I had some interest in politics (although it would be years before I would be come the political junkie I am today). I nearly failed the class, only passing the final exam to pass the course.
But the grades in those 2 courses, and 2 others, ended up not counting in my GPA because in the middle of my second year, I switched to NYU's business school, so those courses were reclassified as free electives - the credits counted, but not the grades.
In my third year, I took a course in Cobol, as I was a Comp Sci major in the business school at the time. I did an extra credit project to get my grade up from a B+ to an A-, but the professor for some reason disregarded my effort and I ended up with a B+, keeping me from making the Deans List. I was furious, and after the next course in my major was not to my liking, I switched majors (to Economics). A good move, because I aced all those courses. So that Cobol professor did me a big favor.
But the worst classes I took were in high school. The first was 9th grade Social Studies. It was Asian and African studies, and many of my honors-level classmates were failing or doing poorly. It was so bad that my mother led a petition drive to oust the teacher for incompetence. The teacher had tenure, so unless he raped a student or burned down the building, the district couldn't oust him. That was the only thing I learned in Social Studies that year.
Another bad class was 11th grade English. The teacher wasn't incompetent, just not suitable for 11th grade (non-honors) English. In a parent-teacher conference, my mom spoke with him and then headed to the guidance office to get me switched out. She told me later that he should be teaching college-level English. My new English class was much better, as I had a friendly, easy-going, and more HS-appropriate teacher.