First thing and most important, you have to go to the parent conferences. With your child, you will likely get an answer about their studies that tries to put them in the best light possible. Maybe they aren’t putting as much effort into what they should be doing as you or their teachers think. Go have that conversation with the teacher, you will get an unfiltered look at exactly what the problem is.
Now that you know what the problem is, the question is how to solve it. If the problem is a lack of effort on the part of your child in the homework department; sit down and do it with them. People tend to try harder when someone is watching them, plus it will give them a sense of accomplishment when mom or dad sees them overcome an educational obstacle.
Study plans are also an excellent idea for a struggling child. For example, say your child spends three hours studying a night. They have five subjects and is not doing well in one of them. What I have done in this circumstance is set a distinct amount of time that the child will spend studying each particular subject, according to how good or bad they feel they are doing.
These are just two ways to help fix a very fixable problem. All you really need to do is just help your child and let them know that you know they are trying their absolute hardest.