What would you do ?

My little sister is in 4rth grade . She came home today looking sad she was saying that his teacher screamed at her for doing her homework to early . I know who this teacher is he has a bad temper and is known to scream at kids and break rulers( something he did long time ago) also supposed he threw a kid inside a trash can bin . I am not happy to see ,hear what my sister is saying cause I know who that teacher is and how he can be . Now I’m telling my mom to talk with the principal so he can calm his shit down.