Hi
Practically you being a parent should address a letter to the school principal demanding TC.A written record is needed , keep the copy of it for your legal purpose
If she refuse or if you have already done it, next step is to reach out to the trustees.
This either you can approach them directly if you have access to them
Orselse
take a legal step sending a legal notice to the school demandingTC and mention about their threat of being intimidated by the school interms of marks and writing a bad conduct certificate
Once this comes on record through a lawyer's notice the principal will stop the harassment
You should ofcourse take care of your job depending on the situation as the personal relationship with the management may affect your work in future