She was the head of the Catholic board and was intensely religious. She rejected all opinions outside of those provided by the church. She happily admitted to the class that:
Family planning works and you shouldn't use contraception of any kind for any reason (She had 9 kids)
She disowned one of those children for being gay
One of her sisters committed suicide and she'd talk about how she's in hell
That she was once a nun that had to stop being a nun for reasons she won't admit but has since repented
A girl in our year got pregnant and at 6 months in had a miscarriage and was out of school for the week. This teacher told the entire class that things happen when you conceive a baby out of sin and the lord was doing his work to take it from her before she polluted it with evil.
She got one of the other teachers fired (My favourite teacher) because she figured out the teacher gave someone a condom and told them to practice safe sex. The teacher she got fired ran the year book so the shitty teacher took that over. The following year the "Cutest Couple of the Year Award" went to a gay couple.
Not only did she remove that from the year book but she prevented the gay couple from being allowed to attend prom.
For someone that pushed a religion of forgiveness and acceptance, she was the most biased, rejecting, and vile person I have ever met.
But what made her a bad teacher? She stopped any critical thought. She didn't teach or inspire learning, she promoted blind following of an ideal she supported. She didn't want students, she wanted followers.