The Forest Queen by Betsy Cornwell
When sixteen-year-old Sylvie’s brother takes over management of their family’s vast estates, Sylvie feels powerless to stop his abuse of the local commoners. Her dearest friend asks her to run away to the woods with him, and soon a host of other villagers join them. Together, they form their own community and fight to right the wrongs perpetrated by the king and his noblemen.