As far as Mitchell's DNA is concerned it should have been found on Jodi because he was with her earlier that day but forensics failed to find it. Maybe they will do better next time?
If I shake your hand John then there is possibly a chance that my DNA will and could be found on your hand (skin cells, sweat), If you shake another persons hand afterwards then the chance of my DNA is very slim indeed as the other person's DNA contaminates mine. Jodi and Luke were together only a short time throughout that day and had no class together, They were in contact with 100's of pupils that day!! chance of Lukes DNA to be found from earlier meetings is zero or end of..
We know all about these independent witnesses john, these were the ones the judge told off for collusion if i am not mistaken, these are the ones who didn't want to come forward to the police with the information they had but had to have been forced by a person in their office. These were the ones who couldn't get anything right i.e the time they left, how they knew about Luke having seen him in a paper even before any photo's appeared.
We know no DNA of Lukes will be found as it would have been found the 1st time, it's the DNA that is there that wasn't properly tested that will hold the real clues.