Not allowed? I think the verse refers to teaching on spiritual matters, in the context of proper position in the Church. My father just took an on-line class taught by a female for the physically disabled to make basic meals. Given the topic, it seemed perfectly natural for a woman to teach.
If you are talking about teaching catechism, then I’d think Confirmation preparation would be an appropriate time to separate the sexes. Let male teachers prepare the boys for Catholic manhood, the female teachers prepare the girls to be Catholic women.
If speaking of academic teaching in school, the time of puberty should be the dividing line. Separating boys and girls should begin in middle school, grades 6-8, and be the norm for the high school grades.
From a strictly pedagogical point of view, it is certain that boys and girls learn differently, and if possible, all but the youngest, grades K-2 or 3 should be taught separately.