The Blushing Harlot by Merry Farmer

It’s nice to be naughty….
After her naughty escapades the night of Lady Charlotte Grey’s engagement party, poor Rebecca Burgess has been shunted off to Miss Dobson’s Finishing School—which everyone knows is little more than a reformatory for young ladies who had sullied their reputation. She would do anything to break free and to find herself in the arms of Bow Street Runner, Nigel Kent, again.
And then the Chandramukhi Diamond is stolen.
Nigel is tasked with investigating the theft of the priceless diamond, but it’s impossible for him to keep his mind on his work when the Rebecca becomes an unlikely ally in his investigation. The diamond was stolen from the house adjoining Miss Dobson’s school, and the network of secret passageways between the buildings allows for more than just eavesdropping.
But when Nigel and Rebecca take their dalliance too far, Miss Dobson might just come between them forever and allow the diamond thief to get away to boot. Can true love rescue Rebecca from horrible treachery?