Over the Easter term, the inky tadpoles changed/ from commas into exclamation marks.
This stanza introduces the theme of change. It is the Easter term, a time of growth and regeneration. Duffy signals this with the mixed metaphor that links developing frogs to writing. The metaphor here is in keeping with the school setting, but also the move from the insignificant "comma" to the bolder (and taller) "exclamation" skillfully reflects the altered atmosphere within the playground. The exclamation mark also anticipates the shock the child feels when she is told how she was born. The tadpoles suggest sexual reproduction as well as development and growth.
The growing children are described through the frogs "jumping and croaking away from the lunch queue" – "croaking" could imply their voices breaking. Instead of gold stars and coloured paper we now have a "dunce" and a "rough boy" taking charge and causing havoc. The feeling is the child narrator is exposed. She is no longer in the protective classroom, but outside learning about the facts of life.
Her first reaction is anger: "You kicked him". The short sentence here evokes her disbelief and perhaps her fear of the unknown.
stared/ at your parents, appalled, when you got back home.
The use of "appalled” parenthesis places the word in the middle of the line, adding emphasis to her horror as her familiar and safe world disintegrates in front of her eyes.