Like Carrot Juice on a Cupcake by Julie Sternberg illustrated by Matthew Cordell*

#3 in the Eleanor series
A new girl Ainsley begins 4th grade.  Pearl, Eleanor's best friend, is assigned to be Ainsley's buddy.   As Eleanor fights off jealousy, she does something mean to hurt both Pearl and Ainsley.
*Highly recommend series.
Strategy: connections, inferring-character's feelings