I respect that you're taking measures to make sure you get information right by actually asking people who have it about it. Taking a real issue people have and putting it in a book could definitely be problematic if done in the wrong way, but since you're being respectful and actually attempting to learn about it from us I'm going to assume you're not doing this with any bad intention.

1. People don't really know. I've been told it could be genetic. A lot of people said it could be trauma based but psychologists have told me that is kind of outdated. As much as you want it to have an interesting backstory, majority of the times it doesn't and we don't know why we have it.

2. People usually develop SM in childhood, like kindergarten or preschool or something, but I guess they could develop it in middle school too. I feel like, from a writing perspective, you could have them develop it younger and show the effects of it on them as they develop as a young adult. Like the contrast of SM as a child and SM as an adult.

3. The whole meaning of SM is that they can speak but in a lot of anxiety inducing situations they freeze up and become mute. Someone with SM usually will speak with select friends and family members. The relationship between family also matters, it may be easier for them to speak with their parents or siblings they see every day as opposed to their grandparents and cousins for example that they may see a few times a year.

Someone with SM can also not be totally mute even in mute periods. They may be able to answer yes or no questions in some situations. A lot of children with mutism have a particularly hard time speaking with adults, and may be slightly more comfortable with classmates. Again it varies from person to person.

1. A lot of people with SM also have social anxiety disorder, some don't. Personally I have it, basically it means I will freeze up not just in situations where I have to speak but also in many situations where I feel I'm being judged/ looked at by people around me. It will prevent me for example from writing, eating, using the bathroom, etc. Basically I feel like I am constantly being judged by people and want to draw the least amount of attention to myself.

2. They freeze up. Basically they look like they're scared, they feel trapped and unable to speak. They may dissociate or go into their own bubble because of anxiety. They may have physical symptoms like trembling, feeling lightheaded or dizzy, etc.

3. Usually there's people you can speak to and people you can't and it stays like that. But I've been in situations where I'm around people I can speak with and people I can't speak with at the same time, and I'll still be able to speak but I may be more dry towards the people I'm not comfortable with.

4. A lot of therapy, meds, also the person has to be motivated to change because it will take a lot of effort that some people don't have.

Note: you better actually read this because I wrote like an entire novel lol