I am guessing you misspelled the word "lesson" here, so I changed it in your question before answering. If I am in error, please repost your question.
That said, the historical lesson in this story is that equality of the sexes is important along with the negation of gender stereotyping. This story is about India and how teens (especially girls) are burdened intensely both by arranged marriages and a lack of education. Here is an example of when Koly truly feels that burden: the burden of women.
There were days when my ma took only a bit of rice for herself so that the rest of us--my bapp, my brothers, and I--might have more. "It's one of my days to fast," she would say, as if it were a holy thing, but I knew it was because there was not enough to go around. The day I left home, there would be a little more for everyone else.
As a result of these burdens, Koly challenges the culture of her Indian country. She learns to read and rises above the negative aspects of her beautiful culture. Further, I do mean "beautiful culture" here. India and its traditions should not be neglected. There is significance, beauty, and importance there. Koly thinks so herself, and we can see this as readers if we look closely at Koly's description of her Indian home:
When I looked into the temples, I could see the holy sadhus sitting in long rows, bare chested, their heads shaven, holding sacred lamps, and accompanying their chanting with bells.
This shows that one can challenge the negative aspects of one's culture while still embracing the positive aspects of one's culture.
In conclusion, put simply, there is one main directive: LIVE IN A PLACE OF INEQUALITY? MOVE BEYOND IT OF YOUR OWN VOLITION! There are also, however, two indirect challenges that serve as lessons: GIRLS ARE EQUAL TO BOYS, and EDUCATE THEM ALL!!! This is also an indirect piece of advice.