About their parents , as the first episode I think Emma said they would not understand , and besides it would be very weird If your dad or mum knows , and they just want for their lives to be the least weird possible so ....
and I don't know very much about Australia's climate but I guess it doesn't rain that much their !
They can't go to the bathroom at school that's for sure , so they I don't know maybe will be prepared hmm
as you said Cleo took that job to prove her point , she took the training job I guess cause she loves dolphins I mean she swims with them in the sea , even though it's risky , Cleo with her power can actually protect her self from water , remember that episode of her wearing the jellyfish costume?
same with Australia's climate , or maybe their Area they live in!
For collage they could rent a house or anything , maybe they'll go to the same collage so they all live with each other
they've been mermaids since a lot of time , so they know pretty much about those things! :)