I’ve been teaching group fitness classes for over 20 years, and I always first start with a general reminder to the whole class about form on every exercise (while perhaps looking at the person who needs the correction). I also walk around the room occasionally to monitor and remind them about proper form.
In general, hands on in a group fitness class is not something I generally do (you may in Yoga, etc…). In my group fitness classes I am participating as well as teaching, so my time is spent in front of the class. That being said, if someone was repeatedly needing correction I would pull them aside after class and show them proper form. I may do hands on in my small group classes as those classes I tend to coach and walk around more.
Hope this helps,