It's really common in any discipline for a "warm up" to consist of some half-hearted stretching while gossiping with your classmates about the last GoT episode or how much you drank last night or what cute thing your dog did this morning.
It should not surprise anyone that the only muscles that get warmed up this way are your jaws.
While stretching can be part of an effective warmup, warmups should focus more on *movement.* Getting blood flow to muscles you're about to move, getting your heart rate up, pushing joints through their range of motion to get out any pops, snaps, or creakies, and wiggling out any stiffness.
There's any number of great ways to do this, but Mackenzie's favorite for her silks classes involves forcing everyone to dance to The Fitness Marshall videos, which hits all of the above goals! Her current favorite is an excellent shoulder warmup and a great chance to show the whole room your sweet moves!
Regardless of how you do it, before your next class, stop sitting on your booty and start shakin' it instead!!