Your school marks them, the principal has to sign something official then the school keeps the projects for future reference for the Board of Studies..for like yrs...
I remember my ext hist teacher commenting on how absurd it was that the Board of Studies might catch someone out for plagarising or something several yrs after the hsc... I mean do they suddenly change your hsc marks yrs later and then kick u out of uni several yrs into ur degree...or even after completing one.. lol.. i'd love to see that happen