I know this one lady that puts a recorder in her kids backpack to record there teacher?
You can record them, if they agree to it.
If parents have any worries about how the teacher is treating their kids, if the kids complain about bad and unfair treatment, then they should get a parent-teacher conference and explain their worries. They should ask the teacher to explain their side, too. If there is a lot of trouble, they can ask the headmaster to join.
If you are in the US it is illegal to record someone without there knowledge