I don't think it really should come down to a binary relationship between mental illness and "enjoyable and engaging", though. I do get your point. But, also, it's school. I don't remember always enjoying it or finding it engaging, but that didn't mean I was slipping into mental illness because a class was boring or didn't seem relevant when I was a teenager.

To your point, the model of school hasn't really changed in a long time; but, then again, we do see rates of mental illness changing. So, I don't really know if you can put the blame on the model of schooling in that instance.

As a side note, one thing that has changed in schools is the drop in physical fun, either in the expectations of gym classes, or during recess, or in school sports. It's either study like crazy, or focus on an extracurricular (possibly a sport) as if it's a second job to get into a good college; a lot of kids just aren't having good old physical fun as much as they used to, at least from my observation.