If I were you, I would follow buckthedog's advice to the tee. Being honest and humble will go a long way in traffic court. Drop all excuses and throw yourself on the mercy of the court.
That cop was doing you a big favor. I don't think you should try to get it thrown out on a technicality. That might reflect badly on the officer, and make him less likely to go easy on people just having some fun in the future. Gone are the days when Andy Griffith can blip his siren and shake a finger at you. If you continued on your way and ran over a kid on the next block it would be his ass along with yours.
...and take that note off your mirror before you get another ticket. There is no way you will get off with a warning with that written on your mirror.