rachel clawson which includes making sure her 8-year-old daughter is wearing a mask. clawson says that routine is critical because her kids attend a mesa public school. it is the state's largest district and has no mask mandate in school until after there is a covid outbreak, but they encourage staff and students to wear masks while indoors. >> it is very stressful. we had a few instances already where she had exposures. we thought about switching to other school districts. >> reporter: now, a new cdc study shows she's right to be concerned. researchers analyzed data from nearly a thousand k through 12 public schools in arizona's two most populated counties. some had mask mandates for students and staff, from the first day of classes, others require masks later or not at all. what did they find? schools without mask mandates were three and a half times more likely to have a covid outbreak than schools that started the term with mask mandates. the study's author says these