This pretty much goes in line with what Annie said above. It makes sense.
I have an eight year old who went through this....we tried laxatives...miralax...several doctors...and even a therapist and nithing worked. He woukd poop himself and not care he'd sit in it...or it would fall out if his pants. Finally I got an enema from cvs and used it and it got everything out and it fixed the problem. I now do it twice a year if he gets constipated and has trouble. It was not a fun experiance for either of us and he went through so many clothes and underware until we fixed the issue. I would get phone calls from school to come clean him up and eventually cps came and asked me and my family questions due to the soiling of himself...long story short they closed the case and found nothing wrong. I know I'm not the only one who has experianced this and I hope my story helps. Try the enema...not only did it work but he felt so much better after.
Is he pooping at home a couple of times a day? Even though you are sure he doesn't have constipation, a lot of kids do "hold it" to the point where their colons and lower intestines are pretty dysfunctional. The bathrooms at school are not usually monitored by adults, partly because the school is worried about lawsuits and so does not want to have only one adult in the bathroom, and also because many elementary schools only have about one male staffer so there is no chance to have two of them in the bathrooms to keep order. Even if there is not overt bullying, there is a lot of teasing, and if a kid has to take down his pants to pee there is even a lot of peeking under the stall door and mocking the kid's bare bottom. It wouldn't be surprising if your kid has decided to 'hold it' rather than endure that. Once that becomes a pattern, there can be a lot of impacted fecal matter in the lower intestines and some liquid slipping by, which is why he might not even know if he has any in his pants. Please look up encopresis, and if the symptoms sound familiar, get him to his pediatrician and ask for an X-ray.
Since this behavior is relatively new and your son is 8 years of age, it would be a good idea to get your son evaluated by his pediatrician to determine whether this behavior is being triggered by a medical or an emotional problem. You don't want to discipline him for such behavior if he has limited or no control of it as this would be counterproductive. The best place to start exploring this issue is with the pediatrician.
I've found that a lot of the times in elementary school, the younger kids are afraid to use the bathrooms at recess, etc. Sometimes they just don't want to stop playing to go, or sometimes the fear of the older kids in there will keep them out. And sometimes the teacher won't let them go during class time.
See if your son will admit to any of those things going on. Anyway, I would talk with the teacher and let her know of the problem, and see if she will let him go during class.