I would say elementary students need improvement on sentence construction. They leave out conjunctions, articles, and auxillary verbs.... (However, I will say Korean speakers of English do this a lot generally but are still understandable.)
Some of them don't have the confidence, or have not tried hard enough/ or had the best instruction, to say more than "I am fine and you/ I am great (etc)."
I think the textbook material in elementary school is fun for students but it is not really teaching them more than quick plug-and-use phrases... they are not really learning how to build English sentences this way... But maybe that is what our job should be as middle school teachers. I feel that it is one of my goals for the students here...
I also teach at 3 different elementary schools (all grades, except K), just started in March, so it is interesting to see what material students learn in elementary.
I think unless students go to hagwons, they are not encouraged to / or use, English outside of the 40 - 45 minute class in public school. This is not enough, they need to have practice outside of school... that is why it is so nice to see students around my town and strike up a conversation with them if possible.