It should be obvious both ways. On one hand, the people most dispositionally inclined to skip school are the people most inclined to drop out. It should be fairly obvious there's a causal link too, because if you're missing every fifth day of school you're probably going to fall way behind in your classes and understand less and less.

Still, the magnitude of the effect is surprisingly large, and this information is useful to schools if they're trying to minimise dropout rates.

I wouldn't be entirely sure of your assumptions that missing class makes them fall behind.

An example is myself. I am a high school drop out. I gave up when, in Grade 11, I passed my Math 20 class with an 85% - but had credits withheld (meaning that I either had to retake or had to make them up with some other class) because I missed 60% of the class. This repeated itself in other subjects. I didn't need to go to school, I was still learning the concepts etc, but I would not have graduated without repeating classes (and actually being there for them) because of my attendance record.

I'm also aware that this is anecdotal, but from discussions I've had with other people in similar situations, it's not that I was necessarily unique.