Is it permissible to continue working at a school owned by a Christian woman who attempts to Christianize the teachers and prevents Islamic activities, knowing that the subjects taught are purely secular?

As long as you preserve your religion and your prayers, and you praise Islam in front of the students and encourage them to fast, then your continued presence at work is a door to inviting people to Allah and warding off tribulation from the female students. You must strive in conveying the message and clarifying the truth to the best of your ability. If you can minimize the evil of this woman, then do so.