Is it ok for my ex to keep my son out of school?

My ex husband and his girlfriend kept my son for an extra day this week without my permission and they umed that i was over-reacting. They were suppose to take him to school on Monday and I was to pick him up Monday afternoon. When I got there he wasnt there and I was told by the YMCA that the girlfriend had just left with my son. Highly upset I called my ex asking him what was going on. He told me that he had to go get picked up b/c he acted out in school, ok so why let him go to the Y and then pick him up 10 mins before I got there? didnt make any sense. As the night proceeded and I told them to bring him home, they refused to and said I needed to go get him. Not to mention I called the cops and they told me they couldnt do anything b/c it was a civil court action that I needed to contact the school to see if they tried to contact me, then I needed to contact the courts. After a night of arguing, i went to bed and decided to take care of it the next day. I went to the courthouse and got my divorce papers showing that his dad was to have him 2 days every other week. No other statements on the papers. I'm not hateful, I've actually let them get my son each month for extra days, with no problems. A couple of times we had our disagreements but that was b/c they always ask. Well I got an email from his g/f stating that the next time they had my son and wanted to keep him out of school to go out of town they would w/o my permission and that I couldnt prevent them from doing that. She also kept saying that keeping him out of school was no big deal and that it was my ex's decision to keep him out whenever he wanted. So what do you think? I want to take him back to court but don't have the money right now, so what can I do, is there anything I can do?