I know I struggled with writing when I was in junior high and high school. Now as an adult I find it relatively easy I think. My advice to my junior high school teacher would be to talk to the student (me) about something they enjoy. Whatever that is today I have no idea. Fortnite? Boys? Girls? whatever, ask what the kid is into. Then ask questions about it. Why do they like it? What's so cool about it. What's it like to do. Etc.. After they've talked your ear off for 10 minutes tell them to just write everything they just told you.

No idea if that would work but I feel like most people have no trouble talking about things they enjoy. Could be a movie, a TV show, a sport, science, twitter, instagram, no idea. If my 8th grade teacher had not asked me to write but instead just talked with me about say D&D, something I was into, I'm sure I could have spoken 1000-5000 words on it. The same words I'd have said to another kid I wanted to introduce D&D to them. I believe that would have helped me write sooner.

I also wonder if the internet is a net plus here. People write short tweets but lots of people write long comments. This comment itself is more than I think I ever wrote on a teacher chosen topic in 8th grade.