I noticed a lot of textbooks don't have enough idioms. Particularly on a junior level. It's kind of crazy when you consider how common they are in our daily conversation in the realm of natural English.
So, here are a few that might be of interest to students;
When it rains it pours -> This can be used in both negative and positive situations. We use this to describe that when one thing happens, another or a few events seem to follow. For instance a student came down with COVID 19 and her air conditioning broke down a day later. When it rains, it pours, eh?
Kill two birds with one stone -> This idiom meaning to accomplish two things in one action is easy to imagine. In fact there is a similar idiom in Japanese, as there is for the previous one, so when your students study this, they will say, 'oh, I get it!'
When life gives you lemons, make lemonade. -> This is more of a proverb than an idiom but it is a good way to encourage positive thinking. The basic idea is when you are met with trouble (lemons), you should do you best to turn it into a positive situation (refreshing lemonade).
Take it on the chin -> When you are down or something bad happens to you, accept the situation and don't complain or turn it into a big problem. Be positive even in adversity.
I like to focus on positive expressions and situation as sometimes learning a language can feel like a very negative situation, especially for some beginners.
Teachers can teach idioms by making a guessing game (can you guess the meaning of this idiom), or finding ways to draw these out in the course of discussion in class. Make sure the students make a note and try to quiz them later or provide them with an opportunity to use them in a class.