Amari and the Great Game by B B Alston

After finding her brother and saving the entire supernatural world, Amari Peters is convinced her first full summer as a Junior Agent will be a breeze.

But between the fearsome new Head Minister's strict anti-magician agenda, fierce Junior Agent rivalries, and her brother Quinton's curse steadily worsening, Amari's plate is full. So when the secretive League of Magicians offers her a chance to stand up for magiciankind as its new leader, she declines. She's got enough to worry about! But her refusal allows someone else to step forward, a magician with dangerous plans for the League.