How English is taught in my school> <

I mean,I think English is taught to us very well by the teacher already,and I'm not complaining about how sucks the teachers are. I just dont understand...Why the teachers just teach us grammar but not some slang and idioms? I mean,yes,of course we need to learn the grammar,but spoken English is important too. The teachers always teach us "sleep","excited" but never tell us we can say "hit the sack" instead of saying "sleep". I don't think we can keep a good conversation going with a foriegn friend if we just use the English taught by the school but not saying without using slang or idioms or what.I think we learn English just because we need to communicate with each other. If we talk with foreign friends just like a robort and not saying any slang or idiom,people will probably not wanna talk to you because your tone is stiff and unnatural. Anyway,I don't think we are learning the real English in my school.

If we talk like robots, not using any slang or idioms with our foreign friends, people will probably not want to talk with you because your tone is stiff and unnatural.