Communication has a default. The default is: verbal, real-time, face-to-face, with all the expected social signals in place. If you do that, you're communicating correctly. If you don't — if you're quiet, or need more time, or prefer text, or sometimes can't speak at all — you get judged for the deviation from the default rather than credited for the communication that is actually happening.
The register incident
A teacher, during morning register, in front of the whole class: "not speaking hm??" Said lightly, as if it was nothing. But said loud enough for everyone to hear, pointed directly at the fact that I wasn't speaking, in a room full of peers. I don't know what the intended effect was. I know what the actual effect was.
Selective mutism isn't something you can comment your way out of. Drawing attention to it in front of other people doesn't help. It makes it worse. And it signals to everyone around you that the way you communicate is something to remark on — something unusual, something worth noting out loud, something that requires a reaction.
The broader thing
It's not just that one incident. It's the general experience of having a communication style that doesn't match what people expect and being evaluated negatively for it. Quietness gets read as rudeness, or disengagement, or attitude. Not speaking when you can't gets read as choosing not to. Preferring text gets treated as avoiding something rather than choosing what actually works.
None of those readings are accurate. But they're the ones people default to when they haven't thought about it, and the ones that shape how you get treated.
What good looks like
Good looks like someone who adapts their expectations rather than requiring you to adapt your communication. Someone who finds another way in — writes something down, asks yes/no questions, waits without making the waiting into a thing. People who do that exist. They're just not universal, and in school environments they were not the norm.
Online, in text, this isn't a problem at all. Everything I build, everything I write, everything here — none of it is limited by how I communicate in person. The medium that suits me is available and I use it. The judgment comes from the environments that only accept one kind of communication and treat everything else as a problem to fix.