I'm going to throw this out there as a possible explanation as to the 'careless driving' part.  Please understand that I am NOT questioning your story.
Lots of times when people lose control of their vehicle for dumb reasons - trying to reach something in the opposite floorboard, changing CDs, chatting on the phone, texting, eating, etc. - they claim that they swerved to miss a deer or dog.  Animals do cause a lot of accidents, but they also get blamed for a lot that they didn't cause.  I was behind a friend going home when I was in High School and he was hauling butt, way over the limit and lost control on a straight away going downhill.  Flipped the car several times, was extremely lucky he was only minorly hurt.  As soon as the sheriff arrived he started crying about the deer that had run out in front of him.  There was no deer.  I saw the whole thing.  He was trying anything he could to keep himself out of hot water with his Dad and that was his first attempt.  
I'm sure the police have heard that same story (or a variation) tons of times.  It's not right, but I'd be willing to bet that because you are young and you were not following the guidelines of your license (only to school and back) and you had a passenger (possible distraction) they might have assumed that you made up the deer but really just lost control of the truck.  Hence, careless driving.  
Good luck in court.