Telling someone to smile in a portrait sitting can cause them to tighten up to an extent that recovery is impossible. It can end the sitting. Telling a child to smile and not realizing the child is tightening up can cause a child to be reluctant to be photographed again. It has caused some of the worst parent-child meltdowns that I have seen. I’ve seen some doozies.
Do I ever tell anyone to smile? Yes. After I have an established relationship with an older subject and they are relaxed I will sometimes try to coach a particular expression out of them. This is only with a subject at least in their mid teens and after I have all I think I need. Even with an adult it can end a sitting.
I would encourage parents and teachers to be very careful when telling children to smile. Be very sensitive to how the subject is reacting. Repeating the exhortation, command, request (whatever you call it) when the subject is tightening up will only make things worse. When I see this happening I try to end the sitting immediately, unless we have at least another hour and I can control the person making the smile “request”.
The best way for a subject to look natural is to be natural. That means not trying to smile. Our job as photographers is to elicit an expression, not command one.