Unfortunately, we cannot warm up before the class, as the dojang is already used for another class (and we don't have large facilities). I asked my group to come earlier anyway, and warm by doing the training before their class (it's a children lesson, not to harsh for us

) but they never do so (I think they don't want to be with kids, as they just left this stage...). But if you want, you can come and warm. Or do some laps in the stadium right newt to the dojang (strangely enough, no one does that too ...)
For a 90 min class, we warm up for about 15 min and then we're slowly getting harder and faster, and it's not warming up anymore
For a 120min class, we warm up for about 30-40 min, but we're usually using some 'game' activity with teams and work a little bit of dojang cohesion. And when half the school is trying to win against the other half, with well balanced teams ... it's not warming up anymore, it's war :bangahead:
We usually strech only at the end of class, for 5 minutes after crunches, push-up and this kind of stuff. Sometimes we're streching a little bit during class, but it's more to let time to instructors to put the right exercise in place when it asks for specific equipment (I discovered this little trick when I started to teach)