Phineas breakes the 100 Yard Free Style record for the Devon school which had previously been set by A. Hopkins Parker in 1940 at fifty-three seconds (42). Finny shaves off .7 of a second when he attempts the same feat after breaking into the pool with his friend Gene. Gene gets really excited for his friend, but Phineas always wants to follow competitive sporting rules in an honest fashion. It's kind of like how Phineas feels about telling someone how tall you are; you don't say 6 feet if you are only 5'11".
At Gene's suggestion to tell everyone that Phineas has broken the record, Finny says that they aren't going to tell anyone because there were no witnesses and no official score-keepers. Gene suggests that they go get the coach and try the same feat the next day, but Finny doesn't want to do that because he just wanted to see for himself if he could break the record. Here we see a plimpse into who Finny really is even though his best friend can't see it; Finny is humble and honest.
You have a good answer. While i am not a teacher myself, i am currently reading A Separate Peace. And i believe where you said Finny says dont say you are 6 foot when you are 5'11 is wrong. in the story he says dont say you are 5'9 when you are 5'8.5