Bureau of Justice Statistics
The Bureau of Justice Statistics' mission is, as listed on their website, "to collect, analyze, publish, and disseminate information on crime, criminal offenders, victims of crime, and the operation of justice systems at all levels of government. These data are critical to Federal, State, and local policymakers in combating crime and ensuring that justice is both efficient and evenhanded."
Perhaps the best resource available to K-12 teachers, is the collection of Key Facts, which offers visual representations of statistical trends in criminology. Here, you can find graphs on everything from the number of adults in probation, prison, jail, and parole between 1980 and 2008 to violent crime rates by gender between 1973 and 2008.