The school nurse looks to be in her late twenties, on average.  Standing about 5'6", Mercy Winters has long dark hair that she normally wears piled up onto her head, usually fastened with fancy-looking hair sticks or some other accessory.  She has big brown eyes behind very fashionable looking glasses (her eyes don't look distorted, so the glasses obviously aren't very strong).  She doesn't wear terribly expensive clothing, but dresses professionally, with well made skirts with dressy blouses.  She wears a 3" heels to work (she's actually pretty good at running in them, should anyone see her do so), but generally changes into a comfortable, if stylish, sneaker when she arrives in the school's medical office, as is her habit as a nurse.
 Her make up is fairly light and she wears only a couple of rings (a garnet on right hand, and a gold woven diamond on her left middle finger) and a an amber pendant necklace.

The old tale of the aspects of a woman's life is "Maiden-Mother-Crone".  Mercy seems to switch between each effortlessly.
 When she laughs at a joke, is in a particularly good mood, or just takes her glasses off, Mercy could easily pass for an 18 year old (her nose crinkles a bit when she laughs, as she has a tendancy to giggle, she gets extremely embarrassed when told how adorable that is).  She has boundless energy and a bit of a silly sense of humor.
 When caring for a patient, either medically or by listening to thier problems, she seems to take on a matronly air of either an older sister or a "cool aunt".  She's extremely patient with someone who needs help, being very obviously a person of great empathy and understanding.  When she's in "Auntie Mercy" mode, she seems to be the one person in the world who can help.
 When she's annoyed or dealing with stupidity, however, her eyes narrow and she seems to take on the air of an "old school" grandmother, the type that would make an errant child go to the woods to "cut their own switch".  Though not seeming to have a violent bone in her body, Mercy's stern look has diffused many a confrontation during her time as an ER nurse.  She has no patience for willful ignorance.

Mercy Winters is a walking contradiction.  She is a bit full figured, but walks with the grace and power of an Olympic gymnast.  She is the epitome of the caring, sympathetic health professional, but has a glower that could stun a goat.

In short, her door is always open for any student of teacher.  Just don't waste her time.