One cause of low grades is too many activities or interests outside of school that do not leave enough time for adequate study. A daily schedule could help with making sure all activities and school work are worked in throughout the week.
Lack of motivation or interest can also be a cause of low grades. One way of dealing with this is helping the student set more immediate goals and to tie those in with classroom performance.
Parents can log on to the Student Progress Center to access current grades, attendance, assignments, test scores, transcripts, and discipline reports. Knowing parents are regularly checking this can be a motivational factor for students.
Another cause of low grades is possibly a specific learning disability. The student may be weak in math, reading comprehension, spelling, etc or the student may have a health concern such as hearing or vision deficiency or ADHD that is keeping them from succeeding in the classroom. Counselors can help you with some additional resources to help your student be successful.