I had a student that was a high school graduate, but had practically no actual education. He did not understand the concept of what the zeroes at the end of a number did to that number. (A 1 or 10 or 1000 were nebulous in his mind.) He was in an Electronics Engineering Technician course. What I taught him as new information he grasped immediately and had partial understanding of what it meant and how it fit into the course material. However, anything that required previous knowledge to grasp was difficult of not impossible for him to understand. I slowed the pace of the course in order to help him learn. The other students in the class did their best to help him learn, but in the end, he dropped because he was not "getting" the information in a useful way. He went into another course that had the least reliance on previous knowledge and did rather well. He was not dumb, just uneducated. He was extremely smart, capable of learning new information rapidly, he just could not relate it to the knowledge he should have had but did not.