What is the ruling on the mixing of male and female students under the age of 12?

The mixing of boys and girls in schools is an abomination (munkar) and drags society into many afflictions, corruption, and trials (fitan). It leads to various kinds of evil, even if it occurs among young children. Therefore, it is obligatory to separate the sexes in different stages of education, and men should teach boys while women teach girls, except in cases of extreme necessity.