Dsimon, in the comments on this blog one day remarked that he does not do warm up exercises, that he thinks that doing push-ups during practice is a waste of time and he would rather have the kids practice a technique, even if they do it slowly.
I thought about his remark because I used to get irritated at some clubs where I would train when they would spend 30 minutes or more on exercises - jump rope, running sprints, push ups, sit-ups, squats, you name it.
Here are my thoughts on warm-ups.
If you are spending 20% or more of your practice on warm-ups it's a waste of time. Judo class is not gym. Let them get in shape at home.
For beginning students, spending 10-15% of the practice on warm-ups is not a bad idea, for two reasons. One, most new students are not in very good condition. Very few schools have daily physical education classes and your students are a lot more likely to spend their after school hours playing video games than playing soccer. So, warm-ups are good to gradually get them in condition. Two, they don't really know anything yet, so it's not as if you can have them spend the whole time doing throws and pins if they only know one throw and one pin. That's going to get boring pretty fast.
Your warm-ups shouldn't be one more bit of monotony. Some people don't mind push-ups, sit-ups and other calisthenics. Some hate those exercises. I try to mix up what we do for warm ups.
Warm-ups (and every game you play) should be targeted toward judo. I do push-ups and sit-ups because they build muscles you use in judo. We don't have ladders to do ladder drills, but we put belts on the mat, and they work just as well. (In case you don't know what ladder drills are, you can see a few examples here. I'll try to remember to video some tomorrow that have even more applicability to judo.)
There are judo alternatives to jumping jacks, suicides, etc. For example, you can have one player at each end of the mat and have a third player in the middle running back and forth and doing uchikomi. Yes, I have heard people say that doing uchikomi is bad and teaches you how to not follow through on a throw. I know plenty of people who have won world championship medals (including two in this family) who do uchikomi so I don't buy that it necessarily hampers your judo.
My whole point is that warm-ups should not take away time from judo instruction but rather lead into and supplement it.