hello, i think i can help as i go to a private school and the girls are the exact type of aesthetic you're talking about.
in regards to brands i would recommend us polo assn, Ralph Lauren, Laura Ashley accessories, Dior shawl/scarfs, scotch and soda, Chanel scarfs,Celine, max Mara, KHAITE, Gabrielle Hearst. i personally like Victoria Beckham launch with H&M. in regards to actual clothes you need some staple dresses. like dinner,formals. i would also recommend skirt co-Ords, they're very elegant. some fitted polo shirts are stunning and simple with jeans. if i'm being fairly honest its all in the purse, hair and smell. with perfumes i would say Gucci floral, van cleef oud is my personal favourite, miss Dior, bakharout rouge, Prada smells amazing. hair should be soft and bouncy, smelling delightful of course ( you should get a hair perfume). nails either short and clean with a clear polish, or medium natural length with a tint of pink.
i hope this helps ml.