The sister solution / Trueit, Trudi Strain

Discouraged for the first time by her average-kid status when her little sister skips ahead to her grade and gains instant popularity, Sammi sets aside her jealousy to defend her sibling after a school clique shows its true colors.

Sammi Tremayne, neither popular nor unpopular, neither perfect nor a mess, is shocked when her genius little sister jumps up two grades and becomes her classmate. Sammi has always felt stuck in her little sister's shadow, and now her life is a positive social nightmare.