In someone with Hyperkyphosis (an overly rounded thoracic spine) the area that’s too round is the upper thoracic area. In a C-Curve exercise, such as Teaser on the Floor (Chair) the focus of the rounding should be in the middle thoracic area around T7-9 which is opposite the xiphoid process (at the bottom tip of the sternum). That’s where the “upper abdominals” attach to the ribcage, an area that’s usually weak.
Don’t shy away from C-curve exercises for your kyphotic students, but don’t let them flex the spine where it’s easy for them. They will want to protract their scapulae (pull them apart and forward). Make sure to cue them to keep the collar bones wide, and their shoulder blades closer together. Guide them with your hands (tactile feedback is your friend) to close the front ribs and widen the lower back ribs.
Disclaimer: Double-check with your student that the kyphosis is not a result of low bone density, in which case rounding the thoracic spine is in fact contraindicated.