I'm sure that you've heard about this H2O thing. I found out that it's just some Muggle name for water. I want you to do the following: You tell all the Slytherins, then you proceed to inform all purebloods not in Slytherin.
Any non pure-blood using this Muggle classification should be shown the importance of not messing with purebloods. There is a spell Siccum Tergo, you flicker your wrist in a downward motion, oblique left to right. Any time they drink, wash their hands or generally get in touch with H2O, their skin will get drier until it starts cracking and breaking. Truly unsightly, but hardly fatal if cured in time. Make sure only people who can cast it silently learn it.
Regulus Black
PS This parchment will burn as soon as you're done reading. If you ever tell anyone who taught you the spell, you will never finish school