Let me reassure you: no person has ever lost its first language. That is, no matter how much time you may happen to spend abroad or how much you may study a foreign language, your ability to speak your native language will be intact.
The Korean taught in schools, just like all other languages taught in school, is the form spoken by the older upper middle class people and does not take into account the evolved form of language that students speak. The form that students speak will then be taught in schools years later, and that generation will again accuse youngsters of not speaking their language properly, or of losing it, and so on.
The point is that language evolves, and most of the evolution takes place among junior high and senior high school students. These students will come up with new grammar patterns, new words and so on which will not be taught in schools.
Korea is a socially stratified society, just like the United States and many other countries. Yes their is a lower class in the US even though Americans tend to claim that they all belong to that middle class. As for Korea, there is an upper class, a middle class and a lower class, and the middle class tends to try everything in order to one day belong to that upper class.
During the 1960s, linguists discovered that upper class people had a distinct dialect, which tended to be the one taught in schools in developed countries, and was therefore referred to as the "official language" or as a "language" in opposition to what other classes spoke which tends to be referred to as a dialect.
So when Koreans are not speaking the dialect of the "upper class", society tends to be worried that they are not speaking the language properly and that they are losing it. Society will find every pretext to blame these kids for not speaking the proper dialect: they will call them "lazy", or blame them for spending too much time studying foreign languages.
While speaking the upper class dialect in Korea is a big plus to enter upper class circles, the ability of children and lower class people to speak Korean should not be questioned: it is intact, as grammatical, logical and advanced as any other dialect spoken in Korea, including the upper class dialect.