school zones are again sign posted, so you will know when to slow down and get back upto speed, i.e. signs at the start and end of school zones.

You should be able to get around doing 50kmh, but you wont make much progress, also i believe you can cop(pardon the pun) a fine for doing anything less than 80 in a 100 zone. Also go too slow and you'll feel the wrath of some frustrated perth drivers