Far-Knowing, by Melinda Brasher, e-pub version

After the Chaos Mage traps their mentor within a deadly spell, two apprentice mages set off to find and defeat this menace to the kingdom. But how can these inexperienced young women possibly track down and battle the sort of man who destroys villages with summoned wind or fire just for fun?  They've learned no aggressive magic and never tested themselves against a real enemy.

Kallinesha, still an apprentice after seven years, struggles against her lack of raw magical power, compensating instead with discipline and study.  Daughter of the High Commander, driven relentlessly by the duty in her blood, she knows they can defeat the Chaos Mage and safeguard the kingdom.  

Ista, a commoner and daughter of a city baker, harbors no such delusions. But after only three years of study, her power burns much brighter than Kallinesha's.  The task before them terrifies her, but she loves her mentor too much to let her waste away under the Chaos Mage's spell.  Not if there's the slimmest chance of saving her. 

The two have never been able to get along while studying magic.  Will they be able to work together now when so much is at stake?