1) Remember the info Thunder gave you below. Just to make it more fun though remember metallics (bells/vibes) go sharp with the temperature shift. You will be out of tune with them if it gets cold enough and there's no helping that!
2) Blow warm air constantly. Get kids to keep brass mouthpieces in their hands when you're not playing. Have flutes cover all of the keys and blow warm air through the tube, clarinets and saxes around the mouthpiece to not make sound. If you march any piccolos they can be put under armpits or carefully up coat sleeves.
3) Get the kids to be super cognizant of the issues they're going to encounter. Make them aware of the adjustments they're going to need to make. Be sure slides will be moveable when you need them too. If a slide is stuck out and you need it to go in... not much can be done in warmup!
Mr. Dude is right about the onfield warm-up. It's a large part of why I like having a minute of playing a pre-show lead into the show on the field is just in case of something like this you have some time for kids to get up to pitch properly.
I know some people have turned their A/Cs in their band halls way down to simulate conditions before.
Good luck! Only getting to 67 for us, but I'm sure the thirty degree shift from yesterday in class will be 'fun' tonight!