In this line from Chapter 7, Atticus is not being literal. He is using figurative speech in an attempt to tell Scout to ignore the way that Jem is exaggerating his accounts of how school gets to be more interesting as you get older.
In this passage, Scout is unhappy because she thinks second grade is "grim." Jem does not really help -- he says that you don't learn anything that's any good until you get to sixth grade. At this point, Atticus tells Scout to delete the adjectives. What he means is that Jem is exaggerating -- school is not really pointless until sixth grade. But, at the same time, once you get past the exaggeration, there is a kernel of truth -- school does get more interesting, he says, as you get older.