If she's not sure whether she would want to be a professional actress, then what better way to find out than to be in professional shows and meet the people actually doing it? Far better way of making an educated decision than most kids' experience of being a big fish in a small pond and thinking it's always going to be like that.
It will probably be all glitzy and exciting at first, but then she'll also see the amount of work involved, maybe talk to adults who have more balanced stories to tell, meet plenty of other kids who are better than her and see how hard it is. Then if that's what she wants to do, she at least knows what she's taking on.
Specialist music, dance and performing arts schools often get surprisingly goo academic grades. On the on hand the kids are spending a lot of time practising or performing, but on the other hand that might actually do a lot more for the brain than the usual combination of watching TV then begrudingly reading a textbook. Or maybe the kids just have to be pretty smart to perform at that level in the first place.
This is a no brainer. It's not like they learn much of any importance at school anyway.