Can my employer do this to me?

So, I was full time at my job meaning and I had health insurance through them. For full time employees the insurance only costs $20/month. However, I went back to school and only worked two days a week which made me ineligible for health insurance, which is fine because I was able to get it through my parents. This summer, I went up to working 25 hours a week which made eligible for insurance now, and so they are charging me more since I am still part time so it costs me $200/month. I still have health insurance through my parents, but my job said that I cannot drop my insurance because I didn't opt out of during the enrollment period when I was still working full time. I don't want or need to pay for this insurance, and I'm really upset that they won't let me stop it. My only option is to go back down to two days a week because they do not have any full time positions currently available and I will be returning to school in the fall. Can they legally do this to me or si there any way I can get them to stop?