To be fair, i did the EXACT SAME THING! I missed 2 weeks off school before my first exam, and missed all my mock AS Levels. The Head of Sixth form wasn't pleased but he knew how hard I worked during these 2 weeks and etc. And he knew me quite well as I was always sent to his office when I was naughty in lower school (that's a different story).
But I've always believed don't let anything or anyone hinder your progress. And even though, your school may be really strict, if you get amazing results due to that time off, they can't really say anything. I wonder if your school cares about their punctuality rate, or their pupils' success rate??
I think your future is more important than a few detentions or whatever punishment they deem fit for you wanting to succeed in your education.
Due to my revision and me skipping school, I got 98/100 an A in my Philosophy, which I was predicted a D for... So they didn't say ****-all to me cause I proved them right - REVISION AT HOME IS MUCH MORE EFFECTIVE THAN SCHOOL!