Discipline should be direct, personal, and private. These behaviors are a form of emotional abuse. Any threats, teasing, or demoralizing comments are inappropriate. Also inappropriate are behaviors like pushing, pinching, or pulling hair. Discipline should be consistent and immediate.
Small children have short memories.
Delayed discipline is never effective. It must be immediately enforced. Carefully observe the director and staff members. You mentioned that your child is a challenging child. One additional thing you might want to consider in choosing a preschool or daycare facility for your child is the availability of naps, quiet times, and alone times.