Was the ending all a dream?
I'm guessing the bit from where the 2 girls wake up in the middle of a nicely decorated indian room with food is the point where it is all fantasy and made up from the imagination of Sara.
It's not possible for the renovations to have occured without waking them or at least alerting the headmistress. The doors to the school was locked, the entrance to her attic was locked and they even changed out the bedsheets, bed and pillows that they slept on. Yet nobody noticed a bunch of guys carrying food and other decorative materials in the middle of the night?
Sara is clearly imagining this bit onwards after the 2 girls fell asleep fantasizing about food and clothes. She didin't even do any of the chores the next day so it was definitely her imagination.
I'm guessing Sara eventually wakes up from her sleep/dream to realise it was all fantasy and goes on to be the servant of the school for a long while....and her dad never recovers from the poison and is sent away somewhere else.
This movie reminds me of sucker punch which is a movie about "A young girl is institutionalized by her abusive stepfather. Retreating to an alternative reality as a coping strategy, she envisions a plan which will help her escape from the mental facility."