So, this was in the long-ago, back when I was in my third year of secondary school (ages 15-16, for non-Brits). On one particular day, I kinda didn't feel like going into school - I'm not sure I remember why now, I think I might have been reading a particularly interesting book. My parents weren't about to fall for a simple "I feel ill", but I had a secret weapon.
Steven Spielberg's E.T.
In this film, one of the scenes that had stuck in my teenage head was one in which the main character Elliott was also in this situation of wanting to take a day off school. He did so with a clever ruse - he asked his mom for a thermometer, which he then held up to his incandescent bedside lamp in order to heat it up, so it looked like he had a temperature. If I did the same, I'd be able to convince my parents to let me call in sick.
So I said I felt ill, I asked for a thermometer, held it up to my bedside lamp... and it shattered, raining liquid mercury all over my bedroom floor. Luckily, my bedroom at the time had a laminated floor, so the mercury just kinda skated around on it and collected in pools. Even more luckily, my family was friends with one of the physics teachers at my school, so my parents were able to phone her up and say "so... hypothetically, if, say, a mercury thermometer exploded all over your child's bedroom floor, what would be the best way to deal with it?" We got instructions from her, and managed to clear it up using a dustpan and some folded paper to coax it across the floor, and hopefully none of it seeped into the cracks in the floorboards...
A couple of weeks later, when I went into our physics lab, all the thermometers in the school had mysteriously changed from mercury to digital thermometers...