Child abuse and related fatalities are correlated to age of the child, according to U.S. Department of Health and Human Services data. Younger children are subject to higher rates of abuse and fatalities than older children. The decline in abuse rates as children get older is generally smooth, with no abrupt changes (except for the large drop when the child turns one year old).
Child Abuse by Age 2016
Graph reproduced from Child Maltreatment 2016, p. 19
Child Abuse Fatalities by Age 2016
Graph reproduced from Child Maltreatment 2016, p. 55
In particular, there are no discernible signs in the data that children are protected by mandated child abuse reporting when they enter the public schools and private schools. Child abuse rates continue to drift downwards as the children cross from preschool age to school age, when they enter public schools and presumably would begin being protected by mandated child abuse reporting. We see no drop in abuse rates to indicate protection from child abuse by the school environment.
The child abuse fatalities data highlights the lack of protection a little differently. There is a clear pattern in fatality rates as they asymptotically head lower towards a rough steady state as children move from preschool age to school age. Again, there is no apparent sign that children are suddenly protected from abuse-related fatalities when they start school.
One would assume that mandated reporting confers some level of protection to students. Further research may uncover factors that offset such protection.
But currently, the best data available indicates mandated child abuse reporting does not turn public schools into "safe harbors" from child abuse and related fatalities. The author knows of no data-based studies that indicate mandated abuse reporting does produce public school safe harbors.