To hinge at the hips, or roll up the spine?

Putting all other considerations such as specific injuries aside for the moment, this is a debate that has strong advocates either side. Lets have a little look at the arguments;

The Argument For Hip Hinging:
The most stress is placed on the spine when it is flexed (bent forward) and loaded (lifting). Where problems begin to arise is when too much pressure is put through the discs, and this will depend to a great degree on how tight the hamstrings are, and whether or not the legs are kept fully straight. If the pelvis is unable to tilt forwards, then more of the fold will come from the lower back and the strain on the area will be significantly increased. 
Normally, the lower back should curve inwards, however, in this position the lumbar spine is forced to curve outwards which risks creating anterior disc compression (squeezing the discs at the front). Because of the angle of the body, we are also relying on a small section of core musculature to carry out the move, and we have the whole weight of the upper body, especially the head to factor in.

The Argument For Rolling Up:
As with everything the debate comes down to technique. Yes, with improper activation, especially when we consider an individual may not have experience working their core, then to roll up can put undue strain on the back. However, the world simply doesn't work in perfect alignment! Our bodies need to be strong from all angles, especially those we are more likely to encounter in real life (have you ever dropped something, only to pick it up with a perfect hip hinge?). The lumbar position described above is actually one we end up in after too long sat at desks so the importance of strengthening this area is clear.
To roll up effectively, it is important to think about engaging the entire posterior chain. This begins with activation in the feet, and works strongly with the hamstrings, deep hip flexors and other lumbar stabilisers such as the quadratus lumborum.

The Middle Way.
Why not mix it up? This may be determined by the intention of the class you are teaching, the students that are there, or it may just take your fancy to work one way rather the other one day. The important thing… make sure you coach the correct bracing of the core, an appropriate bend in the knees for the individual and engagement in the entire posterior chain (not just the lumbar!).