The effect of attendance depends on your school really.
A few schools may issue a warning if you skip an activity (e.g. sports carnival) day without a legitimate reason and may start to inflict academic sanctions if this is a repeated event. Not all schools do this though but I have heard some rumours that there are a couple of schools (not sure which ones exactly though) that take their activity day participation seriously.
Also, I'm pretty sure universities don't really care about your attendance unless you probably apply for a scholarship or early entry scheme in which they'll probably look at your report (which has your attendance rate most likely). I'm not sure if they look into it a lot though.