What is the ruling on a woman working as an English as a foreign language teacher?

It is permissible for a woman to work as a teacher if it suits her nature and does not lead to fitnah (temptation/strife), provided that Islamic rulings are observed, such as avoiding khalwah (private seclusion with a non-mahram), obtaining her husband's approval if she is married, and that the teaching is exclusively for girls. As for teaching men or working in a mixed environment, it contains elements of fitnah and corruption that render it impermissible.