Good question: as always it comes down to intention. Will you want to work them physically or mentally? Friday afternoons may not be the best time for a really strong yang practice: it may be better to make it a mellowing yin flow to calm them down for the weekend. In this case you may want to start a bit yang-like and then gently slow it down more and more until they seamlessly slide into shavasana. Even though you'd be starting with yang postures, which is not the ideal way to prepare for a yin practice physiologically, it is a great way to structure a class when the intention is to chill.
It also depends upon your students: if the class is full of Type-A Ashtangis, they may hate you for making them do so much yin: in this case, start with a few yin poses and then move into the heavy-duty yang stuff. They will hate the beginning but when the see how deep they can move into the hard stuff they will love it by the end.
Also consider what your students have been doing all day: if they are desk jockeys and have been sitting for hours at their computers, minimize the number of flexion poses and let them move their spines in the other directions.
There are many posts on this topic in the Forum under
Yin Yoga Postures, Practice and Sequences. Check them out for other ideas.
Cheers
Bernie
