I think the best way to improve middle grade is by hard work and concentration. Again the best way to improve in academics is by training, reading materials and exercises. Grades improvement is also triggered by support from teachers and family members.
Make doing well in school your priority. Fs usually mean work is not being completed - so first, you need to complete and turn in all your assignments. As you are working, try to determine why the assignments are difficult. Do you have trouble comprehending what you are reading? Is it because you are distracted? If so, do your reading somewhere quiet and take notes on what you read. Talk to your parent or teacher and be honest about your problems. If you are honest and really want to do better they can guide you. Good luck!