Jen,
I would suggest reading the school rule book from cover to cover. Many schools now have rules about what you post on social media due to cyber bullying. It doesn't matter if it is posted using your own phone, you can be held accountable if it is content deemed inappropriate by the school. My sister had a similar issue with my 13 year old niece. My 13 year old niece (along with some other school friends) were on kik using foul language. A parent brought in screenshots & everyone involved got suspended - it didn't matter that this occurred outside of school grounds or hours.
The least of your worries is the locked iPad. Since it is owned by the state the school can decide how they want to regulate it. So they are within their right to do that. When your daughter downloaded an unauthorized app, she brought the negative attention to herself.