TV and video games don’t affect school grades much when they’re indulged in over the weekend. But, according to a new study, those activities certainly do negatively impact school performance when they’re permitted on school nights. Dr. Iman Sharif of Children’s Hospital at Montefiore in the Bronx, co-author of the study, also found that children whose parents allowed them to watch R-rated movies did worse in class. That correlation was especially strong with boys. The study appears in the October issue of Pediatrics.