It's bad to not be honest about your grades.
Lenny gets his report card which is all B's and comes home when his mom asks him how he did in school this semester.
Lenny wants to answer his mom's question.
Lenny tells his mom that he got all B's as it said on the report card.
Lenny tells his mom that he got one B but otherwise he got straight A's.
Lenny's mom tells him it's not that bad but that he could work harder to improve which he agrees with.
Lenny's mom falsely believes that Lenny is doing OK and doesn't check up on him leading Lenny to fall behind at school.