In my beginning art classes (HS), the issue of cleaning up happens the
first day, so I look out at them and say "Oh, do YOU think it says MOM
on my forehead? Oh, NOooo, you are reading that sign upside down, it
says WOW!"...Sorry, but I don't clean up after you like your mother
does".
While my room is far from spotless (and THAT is an understatement), I
try to impress upon them to leave their space cleaner than they found
it, as a courtesy to the next person, and that is the prevailing
theory. Works for me and them.