If he can't remain professional in his dealings with her and her children at school, it's a problem. If she or her children expect. or receive, some sort of special treatment, it's a problem.
I could very well see some of the parents going to the school board on it if any conflict of interest is perceived.
As was said, legally, not a problem, although the potential could be there.
He will have to be very careful not to overstep the boundaries of what is considered appropriate given his position, and that can be objective.