No,she was definately in the wrong!!
At my son's school if a child is off sick with,for example,vomitting and diarrhea,then that child has to be free of symptoms for 48 hrs before they can go back to school.
When my son was diagnosed with chicken pox(nearly 2 yrs ago now)i informed the school as soon as i could so that they could let the parents of the kids in his class know.