They just have to jump up and try to slap them. In sixth grade that may be hard. By eighth grade many of them can do it. They may have grown a foot in one year. They begin to have waists and muscles and hair in new places.
Hormones are on the loose.
As a result, kids often alternate between periods of high energy and periods of laziness. Both extremes may bother parents. Sometimes they seem to be almost frantic. On the other hand, adults resent the 'laziness' that they see and urge their youngster to do something. To the adult, the youngster's behavior is out of line.