The grades thing works fine so long as deep down you are OK with the piercing.

I wanted my ears pierced and my Mum said I could get it done when I passed 'School C' (short for school certificate - an exam we used to sit when we were 15 or 16). I got good grades (in 6 subjects although that will ony mean anything to Kiwis) and duly got my ears pierced.

My little sister then wanted her ears done but was told 'the house rule is you can get your ears pierced when you pass School C'

all well and good - until my brother passed his School C - came home and said 'I want my ear pierced' - to give Mum and Dad credit they let him do it.