I lied to my parents that I have an important test so I wouldn't have to go to a school trip. They found out, what do I do?

So today (Wednesday in my country) I was supposed to go on a school trip. I hate trips, and I feel very tired and stressed lately so I thought I could enjoy from a nice vacation day instead of going, so yesterday I told my parents that I have an important math test on Thursday and that I want to stay in home and learn (Technically, there was supposed to be a test, but it was postponed some days ago).

About an hour ago (9:00 AM in my country), my mom called me and told me that my teacher called her to ask why I didn't come, and she told him about the test. He told her that the test was postponed for technical reasons.

She told me to call him and check what happened.

What do I do now? She'll probably want answers when she'll come home. While it will probably be easy to make her believe that the test was postponed, my father is far more skeptical and untrusting than her. What do I do now? If they'll discover that I intentionally lied to them they would take my computer and cellphone for some time as punishment, and they will not trust me! How can I convince them that I didn't know the test was postponed (I really do intend to practice math today through, because the test is scheduled to be in the next week).