Hello... I have a quick question
This year I started high school as a 9th grader but I failed one core class... does that mean I failed the school year?

No.

Explanation:

If it is a core class, like Science or Math or ELA you will most likely retake the course during the next year or during summer.

mark twain, real name samuel langhorne clemens, was an american writer, humorist, entrepreneur, publisher, and lecturer. among his novels are the adventures of tom sawyer and its sequel, the adventures of huckleberry finn, the latter often called "the great american novel". wikipedia