Since many modern readers will be unfamiliar with them, we hope you will find it useful to have several posts and links about this and other prayers and expressions in the book.
Sub tuum Presidium Greek papyrus c. 250-280 AD
The Sub tuum praesidium may be the oldest Christian prayer dedicated to the Blessed Virgin Mary, with this Greek papyrus dated to around 250-280 AD.
An English translation of the Greek could be:
Under your mercy we take refuge,
Mother of God!
Our prayers, do not despise in necessities,
but from the danger deliver us,
only pure,
only blessed.
In the book, the class recites the prayer at the end of the school day, for a safe journey home. This is how it might have been sung by the sisters after night prayer:
The Sub tuum praesidium is used as an antiphon at the Nunc Dimittis for Compline of the Little Office of the Blessed Virgin. Here is the plain-song of tone VII, from the Roman Antiphonal of 1912.
For more information, here is an interesting article by Henri de Villiers about the Sub tuum Praesidium.