It would be ideal if students warmed up on their own, the class time could be spent on learning. Unfortunatly to train properly you need 1 1/2 to 2 hours minimum. IMHO 1 hour classes are totally too short. I spend 25-35 minutes just with passive stretching. I spend an hour on "class" (either kicking, stand up grappling, ground work, hand work or combinations of all) Then I do conditioning (about 15-20 minutes). Then if time, another 5-10 minutes of dynamic stretching.
Regular class at a typical HRD dojang warm up and light stretching is about 15-20 minutes.
There is a big difference between warm up and conditioning, and most schools do the combo thing only so no one is really in good fighting shape.
If an instructor is not doing warm ups, students will get injured.