In my 2nd year middle school class the textbook is introducing some idiomatic expressions (a lemon, a busy bee, why the long face, etc). During my time in Korea, there seems to be a love of learning idiomatic expressions.
In my current school, students are having problems with sentence formation. On Friday I gave the students a list of foods and they had to tell me which one didn’t belong. One grouping was oranges, grapes, limes, lemons. Most groups picked grapes. When asked why, a student replied, “Grapes sweet is. No, no, no. Grapes is sweet.” While I was pleased they were able to self-correct word order, there still is the problem with conjugating! Perhaps now is not the time to introduce Break a leg!
Speaking of Break a leg!, I often ride the bus with a few students to school. One of them is quite chatty. Sadly, he usually gets so excited to tell me something that I have a very difficult time communicating with him. He recently told me the story of ghost student which involved a broken spoon, ribs, and something else. All told with great enthusiasm. When we finally arrived at school he said, “Everyday good day, teacher. Break a leg!”.
I realize idiomatic expressions are desired by language learners, but I think the basics should be mastered before these extremely limited phrases and expressions be introduced.
thoughtsonesl 
I’m not sure grapes is the odd one out… unless one has never learned the idiom “sour grapes”!
Also, while you can go overboard, idioms are very useful–they can teach vocab as well the idiom itself. You realize I don’t mean one might actually jump off of a ship into the water, “go overboard” is just an idiom.
Assuming a sufficiency of grammar, idioms are a great way to increase fluency, since native speakers use the ALL the time, without even realizing it.
Just my two cents’ worth, whatever the hell that means.