school for girls from wealthy and famous families
Refers to an elite private school, often with a refined or sheltered image, where daughters of high-status families are educated.
She attends a famous school for young ladies from wealthy families.
At that elite girls' school, etiquette is strictly taught.
お嬢様 means 'young lady from a good family'; お嬢様学校 is the school such young ladies attend.
女子校 is a general girls' school, while お嬢様学校 specifically implies an elite, high-society institution.
Compound of お嬢様 (young lady from a good family) + 学校 (school).