To be honest I am still careful my mum doesnt walk in on me masturbating, but I know she must know I do it because all guys and girls our age do! Even adults do it! Plus there is bound to be a few spunk stains on the sheets from the last 800 sessions or so over the years ive been doing it haha.
Basically what im trying to say is, dont be so worried about your parents finding out. Obviously you dont want them seeing you actually do it but the chances are they already know you do! Its not a crime, Im sure your parents arent bothered either!