A lot of folks (medical) classify cold sores as developed from the HSV-1 virus and are both contagious. I hate to say to use your judgement, but you often do have to when conducting skin checks. If the sore is open or oozing, I'm likely going to say you can't compete. I don't know what the stuff is and I'm not taking chances of allowing more athletes to pick something up or myself. While medical doctors are not normally designated for high school events, you can always consult with the trainer for their opinion. They fall under the medical professional category in most states. I would also say that the coach/parents should see this skin condition and get the wrestler to see a doctor and get the form filled out. If the doctor doesn't feel it is contagious, they can annotate that on the form and the wrestler can compete. I would say in general, but not always, when the (a) sore is crusted over, the contagious part is pretty much over.
Bottom line, is I'm not a doctor and most of this stuff, other than ringworm, is hard to differentiate. If I'm not sure, I'll err on the side of overall safety of ALL athletes, not the one that couldn't get a skin note filled out. This is what I hate about officiating. We are often put into a position to make rulings on things we have absolutely no idea what we are looking at. We are responsible as the person to clear or not to clear a wrestler that may or may not have something communicable.
Below is a slide show of some skin disorders that a dermatologist gave our association a few years ago.
http://matref0.tripod.com/Articles/Wrestlingskindiseases.pdf