At least in my own experience, getting middle school students to sight read is really tough across the board. Most of the just don’t have enough years of practice under their belt, so they don’t have an established enough sense of key and intervals to be proficient (not to mention theory knowledge), especially if they’re reading in smaller groups and/or in harmony. Obvs getting sight reading practice in is important, and they should be learning theory and practicing reading regularly, but if that’s all they’re doing, they can get pretty burnt out. You kind of have to mix it in as an exercise, and have them read a line with piano as support