@Karen I agree. Have a conversation with your son first about his experience in school and how he feel about the teachers the his class.
I would bear in mind that at this age they do lie. My daughter has recently learnt to and she tells my Mum some right tales that are completely made up!
I’m a teacher-from experience a small child wouldn’t make this scenario up-they may misinterpreted something but it seems your child has seen this. If it is true this would be deeply hurtful for a child and would likely feel invalidated. I would definitely speak to the teacher.
No. Be different if he was coming home from school crying. Or was unhappy going to school and leaving. You could ask him if he’s happy at school and likes his teacher.