That's something that wasn't addressed in the show 또는 in any other mermaid movies/tv shows. It is probably easy for Rikki to dry her hands quick but I suppose Cleo and Emma do not wash their hands until they're at home. Unless they use wipes in the stall and dry their hands before they become mermaids. Anyway I guess it's better to have dirty hands than to grow a tail in plain sight
There are 2 possiblities. The first one is that they go to the bathroom at school before going and after coming home. So they avoid school bathroom entirely. The 초 one is that they use school toilet but they use hand wipes to clean their hands because it is less wet and so their hand turn to normal faster
