Sam held on to Dean's sleeve. "He's not worth it, Dean. He didn't even hit me, and you already rescued me or whatever. Can we just go?" The kid's name wasn't even worth remembering, even if he was Sam's latest bully.
The bigger eighth-grader had pinned Sam up against a wall in front of the school as he waited for Dean. (When Dean found out the high school got out ten minutes later than the middle school, there had been much swearing.) Even with his movement restricted, Sam had dodged the bully's poor excuse for a jab. He was about to free himself when he suddenly found himself in a very familiar position. He was shielded by his furious big brother.
Now, he had to keep his big brother from committing unnecessary violence. This was another familiar position. Dean started to lower his arm so Sam would have been successful had the bully not chosen that moment to open his mouth. "Yeah, Dean, go back to that ancient piece of junk of yours. What, is your family too poor to afford a car made in the last 30 years? "
Sam let go of Dean. He knew he wouldn't be able to hold him back, not that he wanted to anymore. Going after him was one thing, but insulting Dean's baby, their home, that was insulting family. Plus, the kid was stupid enough to mouth off to an already pissed-off Dean Winchester, to say nothing of the stupidity of violating the biggest Dean rule, stay away from his car and his brother. He clearly had the intelligence of a lab rat. Lab rats learn best from severe painful consequences. However, Sam kept Dean from punching the kid again after the first punch knocked him down.