I have never been comfortable leading my class through warm ups.
Yea, I know what you mean. The worst is the guy who counts through the warmup as if he's got to get to 10 as fast as he can.
I'm kind of like Jun on this one. I pretty much let people do what they want to do in terms of stretching and then do a gradual warm up in terms of the practice. Of course, beginners means structure. I'll start with something static and slow, move to something with a bit of movement, maybe some rolls (low and mellow) and then move to a more dynamic practice.
When I played basketball I used to do a lot of stretching and it seemed (no scientific study here
) to take a bit out of me and I'd seemly get injured just as much, or more, as when I didn't stretch. I eventually changed what I did and now rarely do a full-on stretch prior to class preferring to just do a gradual warm up.