What is the ruling on the mixing of sexes in elementary schools, and when should they be separated?

Separation between the sexes and prevention of unlawful mixing between them is obligatory during adolescence, because an adolescent's ruling concerning women and lust is like that of an adult. An adolescent is like an adult in terms of looking [at women], and his guardian must prevent him from touching or looking at foreign women, and they must veil themselves from him.