I was wondering what are some difficult high school classes to take.
Quantum Physics
Organic Chemistry
Galois Theory
But in general, "hardest"is relative, and classes available to take depends upon the HS.
Can you provide some context - like, what year are you in and what are you thinking of?
Depends on your school and your interests. AP Language and AP Calc BC are very difficult in my school but it varies at each school and how teachers weight grades (in those two classes 80-100% of the grade is from tests).
There’s complexity of the subject matter and high levels/expectations in the workload. Sometimes they’re found in the same course, and sometimes not. Either one can make a class “difficult”. And they vary from school to school, and even from teacher to teacher.
If you’re looking for classes that impress AOs, start with the requirements listed on the college web page, and take the most rigorous version of those you can do well in.
Don’t take a hard class you aren’t prepared for. That will not make you look good to colleges.
Take the honors version of a class if you can. Then take the AP version if you can.