Even if you dont get along you can learn to not respond to any negatives offered especially if there is split custody or enough visitation that in both homes school homework will be done. You could talk to your ex and ask your ex to re direct his new mate to either say nothing or remained focused on the child wich is the whole point of parent teacher confs . Legally the step parent is just that a step parent with zero rights to parenting decisions BUT like it or not this person is going to be involved in many things as a family unit, If you can learn to set aside and ignore any behaviour not appropriate from the step parent and keep focused on the child you share not only will your child do better BUT your child will never be put on the spot by bad behaviour of the adults.