... yeah. When you've got a class of 20+ and an hour, there's only so much a teacher can do, and it's incredibly easy for some or many of the students to just get... left behind. From what I can remember hearing of mi madre picking info up, studies at the time were showing quality of education starts dropping somewhere in the early twenties, and gets progressively worse as class size increases.
In contrast, when there's, say, ten students in the room at any one time, and they're there for 2-3 hours in one go (and then, y'know, maybe go home for lunch or go chill at the library for an hour or something then come back), there's considerably more options available to a teacher in regards to... everything, really. When the student can just stop for 10-15 minutes, maybe read, listen to music, browse the net or go for a walk or something without completely borking up the teacher's jive, things are just... better. And that gets easier to do the smaller the class, from what I've seen -- since instruction is more personalized, less people are inconvenienced if the teacher stops to catch someone back up or whatev'. And so the teacher actually can. They've got time for it, unlike with a normal class.
In the case of the particular school I went to, it was also very easy to, y'know, focus. One day on one subject, instead of six subjects in one day with interspersed interruptions and a great deal of distractions. Stuff like that. It was smaller than many adult schools, even the others in the local area, but from what I picked up you got (could get, anyway) similar deals in the larger local ones and even larger urban ones. It was just very different from high school, and much more pleasing to my, at least, sensibilities.