The entire spectrum of high school math curriculum is fully accessible to most all students of average intelligence if they are sufficiently motivated and are properly taught.
Most often when a student begins to struggle in a math course it is because they have missed establishing a core set of fundamentals concepts that is a subset of other concepts and intertwines throughout that particular course. Struggling students mistakenly see no familiarity from one concept to the next.
Most often these fundamentals are easily re-introduced and re-established with a skilled tutor. Often is as much as a single tutoring session. As an experienced teacher, I can quickly identify and remediate this need. More importantly, I strive to give the student the perspective to recognize how these building blocks are repeatedly being applied in their course. As the student begins to recognize this, it makes the course much less intimidating. This begins the process of build a strong sense of self-confidence in their ability to succeed in the class. They also have the opportunity to see how the building block concept apply more generally in the learning process.
These core fundamentals will be found in every math course. For example in Algebra, being adept at manipulating and solving equations. Again in Algebra, being able to factor polynomials is a crucial fundamental algebra skill that, if mastered, leads to success over several chapters. This concept is often poorly taught. If factoring is not mastered, this will lead to frustration and plummeting self-confidence over several weeks. Homework becomes a dreaded nightly chore as well as an unsolved mystery. Quizzes and tests are disasters. The poor grades that follow is a source of conflicts with teachers and parents. A general aversion towards math begins to set in. This is also the beginning of the lowering of their expectations of success and their level of motivation in their study of math as it was just getting started. Lower level, less challenging math classes are recommended and scheduled for the coming years. If so, the student will miss the many benefits that would have been realized by accepting and realizing success with other typically more motivated students found in the more challenging math classes.