Hi, ljdback, 

Thanks for reaching out to the forum with this difficult behavior.  You likely have already narrowed down the reason or root of this behavior, but if this question is still lingering, I'd start here since the intervention will be dependent upon the best analysis of the "why".  

Possible Why's (not a comprehensive list):

• Medical/health:
  • Physiological
  • Hormone/chemical related
  • Medication side effect
  • Etc.
• Mental/psychological :
  • Anxiety
  • PTSD
  • ADD/ADHD
  • ODD (oppositional defiant disorder)
  • CD (conduct disorder)
  • Etc.
• Functional behavior:
  • Attention seeking
  • Avoidence
  • Coping
  • Fear
  • Control
  • Stress
  • Anger
  • Power struggle
  • Etc.
• Abuse:  deliberate and involuntary urinating and defecating can be signs of physical/sexual abuse.  This is a highly delicate and sensitive topic, so if this was a real suspicion, you'd want to work with your school's psychologist, behavior support staff, and administrator to address this suspicion.
• Comorbidity of any of these above

I'm going to throw out some questions to get a better sense of what interventions and results there have been thus far, but please don't feel like you have to reply to any of these.  Give whatever insights you are comfortable giving while maintaining anonymity.

What is the age range?  (early elementary, late elementary, middle school, etc?)

Are there any learning deficits?

Is there an analysis yet of if one or more of these areas seems to be the root of the behavior?  Has a medical/health problem been ruled out yet?

Has an FBA (functional behavior assessment) been carried out yet?  And if so is there a BIP (behavior intervention plan) in place?  If there is a BIP in place, what intervention(s) have been tried so far and was there any change in the target behavior relative to the intervention(s)? 

Does this behavior occur the same out of school in other settings, like home, stores, car, park, etc?

Please anyone else on the forum here jump in with your insights and questions!