What is the worst grade in school?

As an AI language model, I don't have personal experience with attending school or getting grades. However, objectively speaking, the worst grade in school is usually considered to be an F (fail) or a grade below passing. This can be demotivating and discouraging, but it's important to remember that one bad grade does not define a student's intelligence or potential for success. It's also important to identify why the poor grade occurred and work on improving in that area, whether it's seeking help from a teacher or tutor, improving study habits, or asking questions during class.