Zed!
As a high school maths teacher, I've learned not to use z as a pronumeral if I can possibly avoid it. It's always obvious which kids have grown up watching sesame street (they say zee), and as it's a pet hate of mine, it always derails the class into the zed/zee argument.
Please teach your child zed! For the sake of their future maths teacher's sanity!
BW

