So basically when you turn 16 you got your concession verification card thing (the blue card with your name and school name on the back) which would have an expiry date. Essentially, if you turn 18 that year, and you graduate, the school won't be renewing your card and so it will end after the expiration date on the card (if it's before your graduation) or when you graduate (if you graduate before it expires), but you could still use it assuming that they don't really check whether you graduated or not (for your green child opal card). If the concession validation card expires, you will have to use an adult opal card until you get your uni details and have uni send details through to opal after you enrolled so that you can get a university concession card, which if I remember properly, is equivalent to a child opal card as it deducts half the standard adult fare when traveling, which is consistent with a standard green child opal card's fare.