You will stay in D class until you get four (4) punches or declare yourself
up into another class. If you do not receive any punches but have shot more than 300 targets before the end of the year, you are eligible to be reclassified down to E by the NSCA. Also, if you manage to shoot more than 1,000 targets before the end of the year, have not earned a punch, and feel that you are uncompetitive, you may ask for a reclassification.
Actually, you only start in D class if you have not yet shot any sort of registered targets. If you have shot ATA or NSSA registered targets, read the rule book as to where you should start.
http://www.mynsca.com/image/downloads/2009 Rule book.pdf