The purpose of this retrospective study was to describe the magnitude, nature, and costs of assault injuries among teachers, a high-risk group. Review of assault cases (N = 341) was based on internal and case management vendor records. The 3-year assault rate was 3.24 per 1,000 teachers. Assaults were predominately physical, occurring during classroom disputes and while restraining students. Assaults accounted for 2,480 lost workdays and more than $714,000 in lost wages and case management fees. The most costly injuries on a per-case basis were strain-sprain injuries resulting in lost workdays and fractures not resulting in lost workdays. Less than 20% of the assaults accounted for all of the lost workdays and most of the case costs. Study variables accounted for 13% of case costs. Occupational health nurses should collaborate with school personnel, administrators, and parents to develop prevention programs, which may include de-escalation techniques for teachers as well as case management strategies.