Spanish…same reason: too much rote memorization.
Architectural drawing. We were required to take one art elective. I chose this one because it had some relation to science and engineering. I discovered that I can’t draw for beans, even with the help of mechanical aids.
Chemistry I and II. The course work was hard and the pace fast. The teacher also was tough as nails and one of those types that could look through you with a glare that I still recall years later.
The class started on the top of the minute and if you came to his class late he would stop speaking mid-sentence and stand there and stare at you as you took your seat and get settled. Then after another 10-15 seconds or so (that seemed like a minute) he would ask “May we continue now” or the like. That was one teacher everyone made a point to be there early for.
Had the same guy for Physics also.
Ditto. Pointless class. Came away with some knowledge of how words were spelled in Spanish, no concept of pronunciation.
Shop. I was great at mechanical drawing, but I couldn’t pick up a tool if I dropped it on the floor.
And Gym. I actually hated Mondays, Wednesdays and Fridays, all day, because those were boys’ gym days.
Jazz band. I love jazz but couldn’t play it well. Plus, the teacher was addicted to cocaine and was a complete jerk.
Back when I was in high school, there was no such thing as AP, not that it matters. My toughest course was PE. I’d gone to a Catholic school for 8 years, and we had ZERO physical education. Come grade 9, I was in a group of girls who’d been doing softball and basketball and gymnastics and field hockey and all the rest for years already, and I had no skills, nor did I know rules or terminology. I think I passed simply because I didn’t cause trouble and I tried.
Academics were occasionally challenging, but not nearly as tough as trying to sink a free throw.
Phys Ed. I like how they claimed that you don’t need to be athletic to get a good mark in Phys Ed, so presumably it was just sheer coincidence that the athletic kids got good marks and the non-athletic kids got poor marks.
AP Calculus. I can’t believe I made a 4 on that exam - I remember literally zero Calculus. (Except that you get a point on the exam for taking the derivative, so don’t ever forget to write down that step.)
Out of curiosity, I looked up AP - apparently it did exist when I was in school, but it wasn’t too widespread, and it definitely didn’t exist at my high school. The most we had were Honors classes and I was in most of them, but it’s pretty sad when they’re still teaching basic grammar in Honors 12th grade English.
So there ya go. 
“Math 30” in the Edmonton, Alberta school system.
Math 30 is the grade 12 (the highest grade in high school) math class. Due to how advanced it was it was optional and not required to graduate high school. Being in the habit of always taking a math class every year, I signed up for it anyways. Then I failed it. Then took it a year later at an adult high school-type place, and was passed but probably only based on effort and not ability.
Math 30 consists of introductory calculus, advanced trigonometry, and advanced probability chapters among others.
Ever seen something called a “unit circle”? Neither had I before Math 30, and never have since. Totally useless course except for University track science students with a math major.
By my senior year I had already passed Algebra and Geometry classes, had a choice between Algebra II or a combo (1 each semester) of Trigometry and Solid Geometry. Went for the latter and while I passed with B’s and C’s, they were the hardest work I had to do that year.
Philosophy in Literature. Oh, I hated that class! I get allegory and parables, but “hidden” reading meaning into novels is just not something I care to do. Unless expressly presented otherwise (see allegory and parable reference) I tend to take the story at face value. Want to know why the author did something a particular way? Hell, I dunno - ask them. He’s dead? Well, I guess we’ll never know.
Never have figured out why people have to ruin a decent story by overanalyzing…
Chemistry. The teacher graded on a class average. That meant you could get an 85% on a test, but if it was the lowest score in the class, it was an F. That was usually me.
After 3 weeks of busting my ass, and making a D-, I asked to transfer to a different class. It was too late to get into another class, so I dropped the teacher who would probably end up failing me, and took the class by correspondence. I still had to bust my ass to do it that way, but I got a B-. If my uncle hadn’t helped me, my grade probably would have been lower. I did really well on the lessons, but I had to go onto the university campus where I took a midterm and final each semester that were proctored, and I had to cram it all into my last year of school after wasting a month with that unhelpful teacher. The exams were really hard, but at least they were graded on a straight scale.
Advanced Chemistry. We didn’t have classes called “AP” even though we did have advanced physics, chem, calculus and so forth.
Anyway, I loved chemistry my sophomore year and was pretty excited about advanced chem my junior year. Turns out it was a real beast. I was never in danger of not passing, but I think it’s the only class I ever got a C in. The teacher was fantastic. Not a nice guy, but he made you learn it. And the same guy taught all the chem, physics and advanced math classes.
Subject-matter was probably Geometry, the proofs specifically. It wasn’t just me: it seemed like every answer was always missing some statement like “Line A is Line A” that was so self-evident, we really had trouble understanding why it needed to be there or when to include it next time.
One English class was made difficult by a particularly challenging teacher, in a good way. No matter how good you were, she fully expected more. As someone who usually skated through class so I could go to the library and really learn something, I had to actually stop and apply myself there. (The next year, I got a perfect score on the essay section of Academic Decathlon. Showing her that was certainly satisfying.)
Spanish I & II; I don’t do well with memorization and I would spend hours trying to memorize those words and when the daily quiz popped up my mind would go blank. I finally switched over to cramming and just writing the words down before they dropped out of my short term memory. Luckily I didn’t need Spanish III in order to get into college.
Phys ed. That would be because of a) the stupid waste of what could have been excellent physical training time; and b) the unrelenting harassment in the locker room.
I took AP classes, but had no trouble with them, because I wanted to take them, because they were interesting. I also took French, which was by far the hardest coursework for me, because I had no desire to take a foreign language.
(I’ve never once bothered with it since; I’d figured that would be the case, and repeatedly pointed it out when asked – both before taking said classes, and while taking said classes – but who listens to pesky kids, with their silly concerns about spending valuable time learning useful information?)
A.P. Biology. The teacher was also an adjunct Biology professor at a local university, and her A.P. Biology course was a mishmash of 1st and 2nd year university biology courses. It was extremely rigorous.