The school received financial support from Sarah Abbot who pledged substantial money, which allowed for loans to begin construction; Sarah Abbot died in 1850 and left a substantial sum for educational purposes.
After mid-century, Abbot faced several challenges: the addition of a public high school in Andover, followed by the challenges of coping with the American Civil War.
In 1853, the first principalship was offered to a woman, and additional monies were raised for the construction of dormitories.
In 1859, the "strong-willed" but "ideologically moderate" McKeen sisters — headmistress Philena and Phebe—exerted strong leadership by adopting a "school-home" approach.
The years were marked by substantial expansion of buildings.
The McKeens fostered the study of French and German and introduced a "systematic oral language program" on a par with that of Harvard University and which "far outdistanced Phillips Academy", which did not offer any modern language instruction until the mid 1870s.
Under their "no-nonsense" leadership, teachers stayed longer, many for ten or more years.

Abbot had always promised order, but the McKeen sisters delivered it in spades.
They loved schedules.
Up at 6:00, breakfast at 6:30, clean your room——and perhaps a teacher's parlor as well.
Though four Irish maids also helped out at $1.98 a week, the McKeens considered house work part of education ... Miss McKeen might ask a girl to help her change guest beds, questioning her the while on Butler's Analogy.
Between 8:00 and 9:00 a.m. older students often climbed the Hill for a Geology lecture while younger ones did their daily calisthenics, but all must be on hand for the Devotions that formally opened the school day.
Before mid-day dinner everyone wore gym suits with pantaloons, and skirts ten inches from the ground. ...
Recitations continued till 3:30, then came Recreation Hours, with time for walking in pairs, studying, mending, croquet, or (after 1886) tennis.
Supper followed; the evening was an alternation of study hours, "half-hours" for individual meditation ... and "quarters" for room-to-room visiting.
Evening Devotions might mean anything from a hymn sing to a prayerful scolding.
Bed at 10:00.

— Susan McIntosh Lloyd, in A Singular School
It was during the late 1800s that the school had a "golden age", according to one view.
The campus was visited by luminaries such as Helen Keller and her teacher Anne Sullivan and Amos Bronson Alcott.
The leadership of Philena and Phebe McKeen was characterized by substantial fundraising and growth.
According to Susan McIntosh Lloyd, Abbot's curriculum "may have surpassed that of Phillips" during these years.
After 1910, the only structures built were "gates".
The school was like a "family" but commanded by women, in which "women and girls could enjoy one another as persons without self-consciousness or shame."

Abbot's women and girls ... One thinks of Victorian women as confined, and so they were.
They could not openly initiate friendship with boys or men; they were expected to hide their interest in sex or to subsume it within coquettish formulae, no matter how interested they really were.
Partly because of this relative isolation from men, however, loving, lasting friendships between women could quietly thrive, modeled often on the close mother-daughter or sister-sister relationships that existed apart from men's affairs.

— Susan McIntosh Lloyd, in A Singular School
The school received financial support from Sarah Abbot who pledged substantial money, which allowed for loans to begin construction; Sarah Abbot died in 1850 and left a substantial sum for educational purposes. After mid-century, Abbot faced several challenges: the addition of a public high school in Andover, followed by the challenges of coping with the American Civil War. In 1853, the first principalship was offered to a woman, and additional monies were raised for the construction of dormitories. In 1859, the "strong-willed" but "ideologically moderate" McKeen sisters — headmistress Philena and Phebe—exerted strong leadership by adopting a "school-home" approach. The years were marked by substantial expansion of buildings. The McKeens fostered the study of French and German and introduced a "systematic oral language program" on a par with that of Harvard University and which "far outdistanced Phillips Academy", which did not offer any modern language instruction until the mid 1870s. Under their "no-nonsense" leadership, teachers stayed longer, many for ten or more years.
Abbot had always promised order, but the McKeen sisters delivered it in spades. They loved schedules. Up at 6:00, breakfast at 6:30, clean your room——and perhaps a teacher's parlor as well. Though four Irish maids also helped out at $1.98 a week, the McKeens considered house work part of education ... Miss McKeen might ask a girl to help her change guest beds, questioning her the while on Butler's Analogy. Between 8:00 and 9:00 a.m. older students often climbed the Hill for a Geology lecture while younger ones did their daily calisthenics, but all must be on hand for the Devotions that formally opened the school day. Before mid-day dinner everyone wore gym suits with pantaloons, and skirts ten inches from the ground. ... Recitations continued till 3:30, then came Recreation Hours, with time for walking in pairs, studying, mending, croquet, or (after 1886) tennis. Supper followed; the evening was an alternation of study hours, "half-hours" for individual meditation ... and "quarters" for room-to-room visiting. Evening Devotions might mean anything from a hymn sing to a prayerful scolding. Bed at 10:00.
— Susan McIntosh Lloyd, in A Singular School
It was during the late 1800s that the school had a "golden age", according to one view. The campus was visited by luminaries such as Helen Keller and her teacher Anne Sullivan and Amos Bronson Alcott. The leadership of Philena and Phebe McKeen was characterized by substantial fundraising and growth. According to Susan McIntosh Lloyd, Abbot's curriculum "may have surpassed that of Phillips" during these years. After 1910, the only structures built were "gates". The school was like a "family" but commanded by women, in which "women and girls could enjoy one another as persons without self-consciousness or shame."
Abbot's women and girls ... One thinks of Victorian women as confined, and so they were. They could not openly initiate friendship with boys or men; they were expected to hide their interest in sex or to subsume it within coquettish formulae, no matter how interested they really were. Partly because of this relative isolation from men, however, loving, lasting friendships between women could quietly thrive, modeled often on the close mother-daughter or sister-sister relationships that existed apart from men's affairs.
— Susan McIntosh Lloyd, in A Singular School
Abbot Academy