High schools primarily rely on grades to assess a student's high school performance. Because grades are often the only objective assessment a student receives, grades are an accurate measure of how well a student has performed according to the standards of the school. Grades don't necessarily reflect the whole picture, and students who have high aptitude in a particular class can still end up with bad grades. A study at the University of California's Center for Studies in Higher Education found that grades are the best predictor of college success, for example.
A student's aptitude for a subject is a fairly good predictor of how well she'll do in a class, particularly if the class is test-intensive. A student who is good at math is more likely to do well in math classes than one who is not. However, a student's ability to learn a subject isn't just a reflection of her innate ability. It also reflects how effective her teachers are. A student who previously excelled in math won't do well in a math class if her teacher doesn't competently explain the material, and if a large number of students do poorly in a particular class, this could indicate a problem with the teacher or material rather than an issue with the students' aptitude.
Tests measure how well a students has learned the material as well as how carefully she has studied. Taking tests, however, is also a skill, and some students struggle with taking tests. They may develop test anxiety the morning of the test and forget all of the information they've learned. A student who does poorly in test-intensive classes may have difficulty with taking tests rather than with learning material.
No matter how smart a student is or how competently she has mastered the material, a poor work ethic can interfere with her grades. Many classes grade students based upon participation, homework completion or ability to meet deadlines. Highly intelligent students who struggle with time management or who are unable to juggle their various commitments may do poorly in classes that require them to meet deadlines or perform daily work. While these poor grades might not reflect a student's aptitude for a particular subject, they often reflect how well a student will do in college. Students who struggle with completing work in high school may continue to do so throughout their educational careers.
Some students face challenges that may interfere with their ability to get good grades. Family difficulties such as abusive parents or divorce, poverty or a chaotic home environment can all lower grades. A study published in "The Review of Higher Education" found that minority students are at a particular disadvantage with grades, and that grades may not be an accurate indicator either of what a student has learned or of a student's future performance. Disabilities can also interfere with performance, particularly when a school does not offer the accommodations a student needs.