I used to skip school constantly. Fortunately for me, it never impacted on my grades.
That said, I don't think its a good idea. I was extremely lucky that I had the knowledge and ability to get the grades I wanted and expected while still skipping school.
I think it's important to understand why you skip school. If you're skipping classes because you're too lazy to show up, think you don't have a duty to be there, or whatever, than you need to have words with yourself- how are you going to get on in the adult world if you can't commit to showing up to work every day? If you tell your boss "yeah, I couldn't be bothered coming in yesterday" you will be sacked- that's just a fact of life.
Aside from that, there are plenty of "good" reasons for skipping school, but it'd save you a lot worry and hassle if you could find another way to deal with those problems.
I used to skip school because I was being bullied by a teacher (long story). Once I could finally admit that, the problem was dealt with, I was put in another class, and it was all dealt with.
In the OP's case, I don't blame you for not showing in. Schools these days seem to be so caught up in wasting their student's time- if you're not there to learn, how can they expect you to show up? If you're done with your work and there's nothing going on, why not knock off early? After all, even worker-bees do that.