According to the SUWS of the Carolinas website, when a child with good behavior starts acting inappropriately or aggressively, it is likely a sign she is struggling academically. Students who are struggling in class might act out their frustrations or failures indirectly by disrupting class, harassing teachers or by getting into fights with other students. Although adolescents will experience strong feelings of defiance and angst through their school years, significant negative changes in behavior are a strong indicator of poor school work.
The amount of homework your child brings home will tell you a lot about his academic performance. No school homework schedule is the same, so you can't look out for a specific amount of work being brought home. Instead, contact your child's teachers to find out how much work should be arriving with your child after school. Once you know a rough amount, you can make observations. If your child is taking a significant amount of time to do only one or two short pieces of work, it's likely she is struggling and might be producing poor work. If your child is not bringing home or doing any homework whatsoever, her grades and performance are likely to be very poor for those subjects.
Other signals of poor school work include radical changes to your child's social circle or day-to-day lifestyle. Students who were previously active in the school community and partook in lots of extra-curricular activities would have shown a dedication to their school. If that changes and your child becomes less and less active with school, it may be that his dedication and attachment to the school has decrease. As a result, the quality of his school work may have worsened too. Any obvious signs of drug abuse, alcohol abuse, crime and depression might be an indication of poor grades. The Higher Education Center website mentions that alcohol abuse contributes directly to students missing classes and failing exams.
Poor school work doesn't always mean your child is partaking in illicit or dangerous activities; poor grades are also an indication of learning difficulties. Learning disabilities and difficulties are not an indication of intelligence; it's actually a problem with the process of concentrating, processing information and understanding concepts. Often these difficulties can remain unnoticed with parents and teachers both failing to pick it up. A steady drop in the quality of your child's schoolwork might well be an indication she has learning difficulties. Discuss the issue with your child's school if you have even the slightest suspicion of it.