Forward bends are as soothing to the mind as they are injurious to the lower back.
Backward bending is an extension of the spine, and forward bending is a flexion of the spine. Extension increases space between the vertebrae, while flexion decreases the space. The only way you can ensure that your students don’t injure their backs in forward bending is to make sure that they are extending their spine while they are bending forward. This requires opening the hamstrings and lifting the bottom of the belly.
Therefore, for students with stiff hamstrings, it is definitely dangerous to do forward bending. While doing forward bends in class, make sure these students in particular hold a belt wrapped around the foot of the extended leg, and tell them to pull on the belt to keep their spine erect while the rest of the class folds forward. If you have a wall rope, have your stiff student do forward bends with feet on the wall, using a belt around the middle rung to pull on, thereby creating traction in the spine and increasing space between the vertebrae.
When you look from the side at a student doing a forward bend, the sacrum should be tipped forward, or else the lower back will suffer strain. The lower back should never be rounded but should remain straight or, ideally, concave. To repeat, the two critical actions in forward bends are the lengthening of the hamstrings, created by moving the sitting bones away from the heels, and the lifting of the bottom of the belly to bring a concavity into the lower back.