Though now we consider child education necessary, it wasn't always that way. During the times of the crusades, it was a privilege. The schools were for the most part run by the church, so it became even more in control of society. But would it have been good for the serfs and other peasants to have been educated back then? There would have been many benefits and many losses to educating lower class people, but just from saying that it is nearly impossible to determine if it would have been worth it or not.
Some good things about educating the lower class people would be things like teaching them Latin so that they could actually know what the bible said instead of just taking the clergy's word for it and getting tricked into paying them money. Also, some women were educated to become nuns, and used their knowledge to weaken the male dominant stereotype.
There would have been, as in all things, losses to the education of the serfs. For example, the church mainly controlled the education, and it was already the greatest power that existed in the known world. The other very large con was that the un-educated people couldn't think for themselves to break free from their lives. Also, there would have been a much greater mind force and more development if all the people were educated.