Pretty sure this is the Circle of Magic books by Tamora Pierce. Specifically, you are discussing the first book. (There are 11 books total in 2 4-book series and 3 separate books.)
From the Wikipedia (modified by me):
The series tells the stories of four 10-year-old children: Sandrilene fa Toren, Trisana Chandler, Daja Kisubo and Briar Moss, known as Sandry, Tris, Daja, and Briar, respectively. They are discovered in various troubled circumstances and brought together by Niklaren Goldeye (referred to as Niko), a powerful mage, who tells them that they are "ambient mages," which means that they use magic derived from the environment. The four youths do not fit in with the other children of the monastic Winding Circle community to which Niko brings them and are put together in a separate cottage, called Discipline. Here they each learn of their hidden talents: Sandry with thread, Tris with weather, Daja with fire and metal, and Briar with plants. Lark, a gentle woman especially attentive to Sandry since she also has thread magic, and Rosethorn, a sharp woman who shares Briar's ability with plants, live with and instruct the children in their magical and personal growth. Also teaching and guiding them is Niko, technically Tris's teacher, but available to all four. Daja is taught by Dedicate Initiate Frostpine, a smith mage. At first it seems that a merchant, a street rat, a noblewoman, and a Trader (a trading race that is often hated by others) will never get along, but an extraordinary circumstance brings them together. Through an earthquake, they realize their full potential and are bound closely together forever. As children skilled in uncommon magics, they struggle to earn the respect of the adults they encounter and to fully understand and control their magical powers.
Sandry's Book, United Kingdom title The Magic in the Weaving (1997) – The four young mages are brought together to learn about their newfound magic. When they are trapped in the midst of an earthquake, they must spin their powers together to survive.
Tris's Book, UK title The Power in the Storm (1998) – Winding Circle is attacked by pirates with a strange new weapon and a spy amongst the mages. The Winding Circle temple faces complete destruction. What will the four do to stop the attack?
Daja's Book, UK title The Fire in the Forging (1998) – Duke Vedris takes the four and their teachers to Gold Ridge Valley in northern Emelan where drought and wild grass fires have been ruining the harvest. Daja finds a Trader caravan and is torn between two fates, to be back with the people she was raised with but who labelled her an outcast, or to stay with her friends and teachers.
Briar's Book, UK title The Healing in the Vine (1999) – When a deadly illness strikes Summersea, the four and their teachers work to stop it. Just as they begin to succeed, the illness strikes back at what Briar loves most.
There are 2 female mentors (and Niko), not just the one. I do recall one of the girls adopts a fledgling, but don't recall any more details than that.
