Here are a few warm-up activities you can use to help your junior high students transition from the normal class mindstate to a state where they are ready to study and use their foreign language. These "any-time" warm-ups require no prep. They are perfect for those times the JTE suddenly asks you to do a warm-up for the class you're walking to right now.
There's a very large school in the prefecture where I live that has a notorious reputation for bad students. It's the junior high I go to every week. Teachers cringe in sympathy when I mention I work there. In the English classes as this school, warm-ups are essential. Students are recalcitrant and sullen. But a good warm-up goes a long way to helping them get out of their funk and become participative class members. As long as I remember the purpose of the warm-up, it tends to work well.
Junior High Warm-Ups, Ready Anytime
Guess the Number
This game might be a bit easy and almost a waste of time for some junior high students but for others, numbers still don't come easily. Also, it's a warm-up - a transition and a short period of time to get the students' brains into English mode. Best for 1st graders at the start of the the school year.
Here's the guess the number game.
Shiritori - Last Letter First Letter
Another game best suited to earlier in the school year but after 1st graders have gone through the alphabet part of the textbook.
Write a word on the board. The last letter of that word becomes the first letter of the next word.
Here's how to play shiritori.
Parachute Game
For 1st graders, you'll need to wait till they've got a number of words under their belt.
Challengers from each team try to translate to Japanese or write in English the word/phrase you call out from the back. The winner cuts lines on other teams parachute.
Here's the parachute game.
Hot Potato - Pass the Eraser
This is a pattern/grammar practice game. Pick a phrase you've recently studied that students can easily change to make original. (Click link to see examples.) Once a student says something, they pass the eraser. After 30 seconds, the timer goes off and whoever still has the eraser is the loser.
Here's the hot potato game.
Criss-Cross
A.K.A. tate-yoko (たてよこ) - is a simple game of answering questions. Everyone standing, you ask a simple question. Whoever answers gets to sit down, as well as the row or column they are sitting in. Whether it is the row or column is up to the student that answered (although expect the class to yell out "suggestions").
See some more variations of criss-cross.