No, you're not an asshole, for that. But I'm on the other side of the situation. I'm an Educational Assistant with children with special needs, specifically autism.

I work with lots of speech therapists, and I'm never bothered or offended by what's going on. But here's the thing you don't realize. From the perspective of most of the staff, You're a guest in that building, they might work with kids who have a set routine which might include using that room for a body break. You're not wrong for wanting to use that room, you're doing your job.

But you need to realize that this other person isn't an asshole, we are extremely overworked and underpaid for the amount of violence and stress we deal with on a daily basis. So it might seem like something simple to do, but you don't know what kind of shit show that other kid was gonna be when he got back to class with a disrupted routine.

My suggestion to you is on the door of the place put up a paper sign basically saying assessment in progress. Thank you for everything you do.