Is it a sin to skip work because I can't go around the time they planned me in?

Hi,

I have a part time job at the McDonalds, and I have to work around 3 PM at McDonalds, my driving lessons begin around 3 PM every week which I also told them, but they told me I have to handle that out with my driving instructor and 2 PM is the only time he’s avalaible on that day. If I ever wanted to make a change of something I had to tell them that 3 weeks ago before I knew.

I had a internship period where I had to work everyday from 9–5, on late Fridays and normal Saturday I would go to work, on Sunday I would go to church.

Would it be considered a mortal sin to lie that I’m sick tomorrow so I can call off and make a change?