I would simply explain that my children are vampires and as the weather is getting warmer and the sun is re-emerging, they have to be taken to the snow immediately otherwise they will start to sparkle and attract the attention of lesser beings.

Actually, I have no idea. My parents wouldn't let me go on school camp in year 10 as I was doing two year 11 subjects and they didn't want me missing those classes, and made me do my work experience in the school holidays for the same reason ... the thought of my parents encouraging anything that would result in any class being missed whatsoever is something I only ever saw in my classmates, and I'm pretty sure that their parents just notified the school that they'd be out for these days.

The kids had to hand in any assignments before they went though, I think... no default extensions allowed.

I think there's some pretty good (non vampire based) suggestions already here, though.

And maybe the boys can confuse their competitors by speaking alternating sentences in Italian.