Have you tried the Murder Most Unladylike series? Mystery series set in a boarding school.
Hilary McKay is an excellent writer - she has a series about the family of a slightly crazy artist. It's written in a modern style (not bogged down with description), but in feel it's quite a traditional, gentle story. The girls are late primary and secondary age, so there's school and friendship and boyfriend issues.
Michelle Magorian's "A Little Love Song" is a wonderful YA. Do check it out first as there is a discreetly written (but very well handled) sex scene in it. It's not a mystery but there is an element of mystery: Two teenage girls are evacuated to the coast during WW2. They're from an upper class family and not accustomed to looking after themselves, but when their chaperone gets called up they have to became independent quickly, learn first aid, make friends with some very 'unsuitable' people (a single mother in 1940, the scandal!!) and find love.
How about some Agatha Christie? They tend to jump into the action/mystery very quickly.
Love Simon is good, Will Grayson Will Grayson too. Both set in modern American high schools, and enjoyable, easy reads.
At that age I also enjoyed the Lone Pine series - it's mystery and adventure set in WW2 and shortly after. A bit like a more grown up Famous Five. Much better written, less problematic by today's standards, and there are eventually (gentle) romances between the main characters.