I've read through some of the responses and thought I'd add my two cents. I honestly feel that punishment should fit the crime. We have had behavioral problems with our 6 yr. old son, who is in kindergarten. His teacher has a color-coded system. For each "bad" thing the child does, they have to pull a card. The first 2 cards are green. The third is yellow, the final one is red. If a red card is sent home, she writes what the behavior(s) are/were that led to that card being pulled, and the parent has to sign it and send it back.
Upon picking my son up from school each day, I ask how is day was. He then responds with how many cards he may or may not have had to pull that day. If he only had to pull one, I praise him, but I ask what behavior led to him pulling that one card. If it's a minor infraction, I let the punishment from the teacher stand. We talk about the behavior and why he shouldn't do that again, and encourage him to do better the following day.
If the behavior was a major issue, then he gets punishment at home, as well as at school. Such as no tv for the night, early bedtime, no games, and even writing that he will not do that particular action again. He kicked another student, at the beginning of the school year, and he had to stay inside for recess, and write out "I will not kick others" multiple times. When he got home, he got spanked, and he had to write "I will not kick others" again, and I sent it to school so his teacher could see that we also dealt with the issue at home.
I am stunned at the lack of parenting in today's children, in school. I just had a conference with my son's teacher, this morning before class started, and was astonished that out of 7 parents who didn't show up for their conference last week, I was the only parent who asked to reschedule. And the only reason I missed it last week was because my son was sick, and I was at home tending to him.
So share with your ex-husband that it pays to be involved with your child's schooling. You have to take a pro-active approach. And you have to be unified in your decisions, or it will just create tension and strife, and your child will not know what is going on. The child will begin to rebel against you, since you seem to be the one with the most sense. Since your child knows that he/she can get away with things because "Dad says it's ok", it's just going to make things worse in the long run, for everyone involved.
I would even suggest you and your ex-husband speaking with your child's teacher, principal, and possibly even the school counselor, so that you can each be on the same page when it comes to your child's education and behavior issues. Good luck.