What can parents do to intervene if they sense that their child is not doing well in school, and might repeat the school year?

first of all they need to find out the reason behind the decreasing by looking to theire friends, are they good or bad ones, then if there is no problem with theire friends, they need to look to the latest events in theire child's lifes wiether if  the school manager has changed or the teacher.Or even some latest family events like marrige, deaths, all that may affect the child's education and this from my experience.