Can a school district 'unofficially' change my son's schedule when its convenient to them, even though he has an IEP and that is not his placement? 

Backstory: my 13 yr old son (2E, autism, anxiety, adhd, dmdd) is currently serving an  inschool suspension until Thursday. But the district wants to keep him self-contained 1:1 with special education teacher Friday as well but not document it as a suspension (probably because he's at 9.5 days now). Self contained is not my son's placement. And they have done this several times before this school year for their convenience without documenting a placement change.

What do I say to the administration when they do this? 

Document everything. How did you find out about this? If someone from the school told you about it, follow up with an email asap confirming your understanding of the conversation and ask that they please let you know right away if you've misunderstood anything. If your child told you about it, still email the school and let them know what you were told and get their perspective. 

What is your child's regular placement? Does your state have any specific rules giving schools flexibility to change placement for some short period of time without holding an IEP meeting to discuss the change of placement? If unsure, you can always reach out to your SEA for clarification. 

If you feel your school is in violation of the IEP, you can always consider your procedural safeguard options (administrative review, mediation/facilitation, complaint, due process).

With the suspensions, has your school done an FBA and does your child have a BIP? 

IDEA allows the school to change placement w/o the IEP team deciding for 10 days.  After 10 days, it's looked upon as a change of placement where the IEP team needs to decide if this is appropriate.