Sorry to disagree with the teachers but I would seriously take him to another pediatrician because it sounds like encopresis, the involuntary passage of stool. Further, you said most times it is little more than a stripe.
I will give you some links. Don't punish him. Don't pull him from school. If it is a medical issue--and I am betting that it is, the school should be your support system.
Most schools are not familiar with the problem so instead of supporting you, they are threatening you?
Here are many links that you can check out.
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002537/
http://kidshealth.org/parent/general/sick/encopresis.html
http://www.mayoclinic.com/health/encopresis/DS00885
You need a good pediatrician that can help you through this. Constipation is the primary cause.
The plug in the colon the new feces is going around is the blockage. Poor kid.
http://encopresistreatment.com/
ETA: A competent pediatrician can approach it in a competent manner, but pediatric gastroenterologists typically deal with it too. Either way, you need a good pediatric doctor. If you read the links and you suspect he has it, look for a referral from a friend or your insurance company. For a treatment plan, it is best to begin by seeing a doctor. I don't think you should medicate him yourself. And make no mistake, any (small) sign of consistent soiling could certainly be encopresis. Just check the links from respected sources above.