It could be peck marks, but it may be some sort of illness also. Thanks for posting a pic, BTW. I don't want to hazard a guess as to what exactly happened there.
So a lot of the time, if there is a sick or injured bird in the flock, the rest of the birds will gang up on him/her. It is totally normal chicken behavior. Personally, I would separate that one and let it heal up, or if it is something besides injury it's a good idea to separate the bird anyway. Yes, reintroduction into the flock will be difficult, but if that one is getting picked on continually it's probably a good idea.
Some of the roosters might need new homes, once you figure out their sex for sure.
Are you sure that's a cockerel? Being over 2 months old I would expect to see more red in the comb and wattles.
With out better pictures I'd be willing to bet that is a pullet.
I watch my young cockerels with the hens and pullets and they will 'gang up' on them in their zeal to 'feel their oats' so to speak. Feathers are grabbed, both on the neck and on the heads. I'm wondering if you have cockerels who are starting to come into their hormones and thus chaos reigns?
I still have to say rooster pen and the sooner the better.