This is when most kids learn to read. Some kids have basic reading skills before kindergarten, and some learn after 2nd grade. But for most, this is the time wh

This is when most kids learn to read. Some kids have basic reading skills before kindergarten, and some learn after 2nd grade. But for most, this is the time when they go from being able to recognize a handful/bunch of words to being able to figure out new words, understand exceptions, and learn about grammar. There's also a decent amount of conceptual math taught in these grades. I wouldn't say that the concepts are 'hard', but they do take repeated exposure for many kids to grasp them and perform them accurately. You could teach the skills in a week, but only very bright kids would immediately pick up on all of it and not need further practice to be on par with kids who attended K-2. For example, it takes repeated exposure and practice for kids to know that a number ending with 4 and a number ending with 6 will always sum to a number ending in 0. Or to know their multiplication tables. Does it take three years to learn this stuff? No. But it can't be crammed into a week or even a month.