I've actually got a few, depending on how you define "worst class"...
The classes that I had the worst grades in were Spelling and Penmanship in elementary school (my mother would joke that my poor handwriting just showed that I was going to be a doctor when I grew up)
The classes that I was bored in the most were 2nd grade math and 9th grade social studies. In the math class, we used this workbook for our homework assignments and the teacher would usually give us time at the end of class to work on your homework. However I breezed through those problems in class, so the teacher had me color in the cartoon animals and whatnot that were in that workbook, so the math class also became a coloring book class. And the summer before 9th grade our family moved from NY to MN... In NY, the 7th and 8th grade social studies courses were on American history and the 9th and 10th grade were in World history; however in MN the classes were switched such that American history was taught in 9th grade. Since I had just spent 2 years doing American history, my family thought it was pointless for me to take it again; we even tried to get the schools to let me take the 8th grade World history class instead so that I'd have some world history under my belt. But the district wouldn't let any student that passed a certain grade to take any classes from that level, so they made me take the American history again. At least after the 1st semester when I had aced all of the tests, the teacher went back to the school and fought to get me out of this class (not only did she claim I knew the material, but also I was skewing the curve when I was consistently getting 3-5 percentage points higher than the next highest student) and so I was allowed to take a 10th grade current events course, which looked at the current news of the day and the history in that region that lead to what was currently going on. So I got a little bit of world history thru that class.
And finally the class that I felt I had done the worst in was Physical Chemistry (P-Chem) in college. P-Chem is a cross between physics and chemistry. It goes into such topics like the mathematical formulas that dictate the shape of the electron orbitals, thermodynamics, and quantum mechanics. I felt that I was so lost when I first started, and after my first test when I had left over half of the questions blank because I had no clue as to how to even start solving the question, I went back to the dorm and was seriously considering dropping the class (which since it was a required class for my chemistry major, meant that I may even need to change majors in my junior year). However when I got back to class on the following Monday, I learned that the entire class was in the same boat. I guess there was some mix-up with the prerequisites for the class; the multi-variable calculus needed for P-Chem was taught in the year after the calc. course stated in the prerequ. requirements. So the teacher actually gave us a crash course in that necessary math, and the rest of the year turned out much better.