When Sammy first starts school in Crisfield, he is, as Gram says, "an angel." His behavior is impeccable; he never gets in any kind of trouble. Although this would seem like a good thing, Dicey and Gram are worried, because this is completely out of character for the normally pugnacious little boy. Sammy is a good child, but he has a temper and a tendency to get into scrapes now and then. For Gram and Dicey to hear that his behavior is perfect in school raises warning flags for them.
When Dicey talks to Sammy, she discovers that he is putting all his efforts into staying out of trouble because he so desperately wants Gram to keep him and his siblings as her family. Deep down inside he feels responsible for their mother leaving them; he thinks that if he had been better behaved, she might have stayed. In order to keep himself from running into conflict in school, Sammy has kept himself aloof from the other kids, who "don't like goody-goodies." He never plays, because he knows he has a temper; he is afraid that if he were ever to get angry, he might explode, causing trouble that might lead Gram to abandon them like their mother did.
Dicey reassures Sammy that his behavior had nothing to do with what happened with their mother, and encourages him to be himself at school. Sammy takes his sister's advice to heart, and begins to interact with the other children in his class, experiencing the normal conflicts inherent in elementary school friendships. When Sammy gets into a fight one day on the bus, Gram and Dicey are worried, but they are also pleased in a way. Sammy is no longer holding everything in; he is back to being his normal, feisty self, and they are glad about that (Chapters 5 and 6).
Gram and Dicey are happy about him getting in a fight because even though he may be acting like an angel he is acting like himself, so in this case they would rather him be in fights than act unlike himself.