That's hard. My son has had a couple bad teachers. Fortunately they changed jobs rather quickly both times. Have you talked to anyone about it above the teacher? Is another class with a different on the same grade level an option?

https://www.autismspeaks.org/expert-opinion/sev...

Readiness and Preparation

Establish a consistent team: Ensure all caregivers (parents, teachers, therapists) are on board with the same plan, language (e.g., always using "potty" instead of interchanging "toilet" and "bathroom"), and schedule to provide consistency.
Prepare the environment: Make the bathroom a comfortable space. Use a small mat to address a dislike of the tile floor, a padded or insert toilet seat for comfort, and a footstool so the child's feet are flat and supported, which helps with muscle relaxation.
Gather materials: Prepare visual schedules, reward systems (special toys or treats reserved only for toilet training success), and specific language or signs for the process. The Autism Speaks ATN/AIR-P Toilet Training Tool Kit is a helpful resource for visual supports.
Key Strategies
Use Visual Supports: Many autistic children are visual learners. Use pictures or an activity board that breaks down each step: pants down, sit on the toilet, use toilet paper, flush (if the sound is not an issue), and wash hands.
Establish a Routine/Schedule: Autistic children thrive on routine. Implement a "clock training" or "time training" schedule, taking the child to the bathroom at regular intervals (e.g., every 30-60 minutes) based on a log of their current patterns.
Use Positive Reinforcement: Immediately and consistently reward any success, even a small effort like sitting on the toilet for a few seconds or a "small dribble" of urine. Rewards should be highly motivating items not available at other times.
Encourage Communication: Teach a simple sign, word, or picture card the child can use to indicate their need to use the toilet.

Address Sensory Issues: Be mindful of sensitivities to the sound of the flush (flush later or cover the automatic sensor), bright lights, or the feeling of cold tile floors..

https://www.autismspeaks.org/blog/bullying-and-...

Key Aspects of Teacher Bullying and Autism

Reporting and Action: Schools have a legal "duty of care" to protect students. If a teacher is bullying a student, parents should document incidents, review the Individualized Education Program (IEP) for safety measures, and report it to school administration.

If you suspect a teacher is bullying an autistic child:
Document everything: Record dates, times, and descriptions of incidents.
Contact School Administration: Formally report the behavior to the principal.
Review IEP/504 Plan: Ensure the plan includes specific protections and safety measures.
Seek External Support: Contact autism advocacy groups or local education authorities if the school does not take action

I'm sorry to hear that your niece is being bullied. and that the teachers are part of that? Do you know the kind of classroom your niece is in ? Is it a special ed class or regular class?

Teachers are part of the bullying