Why did Mr.Prud"homme accept Phineas' explantion for his absence at dinner? What did this reveal about Phineas?

One of the school masters, Mr. Prud'homme, noticed the boys' absence from dinner the previous evening, and pays them a visit to discuss the offense. Finny (aka Phineas) concocts some long-winded but amusing excuse about going swimming, watching the sunset, etc., and how the activities were absolutely necessary in light of their almost being of age for the draft; Mr. Prud'homme is won over by Finny's charm in trying to weasel his way out of trouble, and the master forgets any charges.