Always, always, every day that you are a teacher, have a bag of tricks. That is, in the bag you carry with you to work have enough props to get you through what ever is thrown at you. So a book, some flash cards, a cable to connect your iPod to a stereo, fluffy toy, pack of cards... ask about to see what others carry.
As unbelievable as it sounds, do not be phased by the number of students that you will be faced. A well modelled game will work well irrespective of the number of students you are faced with.
Don't rush the modelling. Think of it as time investment. You don't want to be correcting 500 individual students once the game starts.
Take control of everything and everyone. You will notice that the other teachers in the room (if there are any) have taken a big step backwards when the Principal cast his eye around the room for someone to run the next 30 minutes before choosing you. You can order them about as you wish because they want to look busy. You need chairs? Send other teachers to get them. You want someone to pause the CD when you say so? No problem. They will jump for you. Don't try and do everything yourself.
When modelling ensure that everyone one can clearly see you. Get up on a platform or stage or have everyone seated.
All the following are my tried and tested large scale games. The student numbers refer to the class size we actually have taught them to.
1. Kick Balls At The Staff Game
There is probably a better name for this. Anyways...
Student numbers: 50-280+
Make one big circle with all the students. Depends on size and shape of the space you have. Failing that 2 circles or whatever fits.
To play: Have all the school children make a circle. All the staff are inside the circle. Children must remain in a circle holding hands at all times. On Go they kick the balls at the staff. When a staff member is hit by a ball that teacher is out.
Repeat as long as you like, but stop before the over enthusiastic ones get too excited.
Student numbers: Good for 10+ students. No upper limit for student numbers for this game. You just need roughly even amounts of people in each team. I've done this with 8 classes of 30 kids per class. Again, enlist teachers and mums to help you and take part. Children love tell their mums off for not playing correctly.
Props: If you have 6 teams, you will need at least 6 flash cards.
Have all but one of the teams seated in lines. Stand at the front of one team. Ask them to identify a flash card. Hand it to the first student. They grasp it with both hands and pass it over their head to the student behind them. That student then passes it between their legs with both hands to the student behind them: "under". Be very firm and obvious to all that this is the only way the card can travel to the back of the team. Restart if necessary. Make sure everyone else in the room can see what is going on. When the card gets to the back of the line that student must run to the front of the line and identify it to you or an assistant. He remains at the front for the next go. As long as the majority of children and most other teachers were watching you then you are good to go!
Have all teams stand and give a flash card the first student and on your command Over Under begins. Have other staff police it. Be sure to fix it so that no one teams gets all the points.
You can play it for hours I guess but as per usual quit while they still want more, so 12-15 minutes max.
Student numbers: Good for 20+ students. Works with 350 plus 50 parents and 30 staff.
Students all seated. Raise one finger, then say "one". Children repeat. Do this up to 10. Repeat except you are silent this time. This is to review numbers.
Make fingers into a triangle, square, circle. Check everyone knows what a circle is.
Ask for three volunteer students. If over 100 in attendance ask for 8. Choose your best and brightest.
March about the room with these students stopping suddenly.
Put your hands behind your back, shuffle, then reveal 2 fingers. The children shout "two". They find a partner from the other volunteers and sit down holding hands to make a circle. Praise heavily.
Have the volunteers stand again, not holding hands. March, Stop! Reveal 4 fingers. The volunteers sit in circles of four. Praise.
Ask the crowd who wants to volunteer. Hopefully everyone will want to. Lucky them, because they are all playing now.
If you have up to 30 students on your own you may need to check your maths, a bigger class with extra teachers there is no need to. Involve everyone in the room. I did this one summer school with 300 students, plus the staff, mums, dads and a few grandmothers.
Use these numbers. 3, 5, 6, 10, 15, 50 (or whatever half the group is). For the penultimate round if there are 100 students in the room, write 100 in the board and have them make a circle that big. For the last round choose "1". All children must then sit on their own. If there are hiding places in the room you can follow that with "zero".