Health literacy (HL) plays a key role in explaining health disparities. School nurses (SN) provide health related expertise within the school setting. A positive effect on the HL of children but also their teachers and parents has been suggested by some research, but gaps persist in the available information. As a pilot project, SN, which are not common in German schools, were placed in 28 public elementary and secondary schools in two German states. Children (11+ years, n = 2773), parents (n = 3978) and teachers (n = 420) participated in a 2017 baseline (T0) survey. Data collection was repeated in 2018 (T1). HL was measured using the Health Literacy for School-Aged Children scale (HLSAC) (children) and the European Health Literacy Short Scale (HLS-EU-Q16) (adults). Descriptive and multivariate data analyses were carried out. The HL of all groups increased between T0 and T1. Low child HL decreased from 17.9% to 14.9%. Problematic and inadequate HL dropped from 43.8% to 38.8% among parents and from 49.9% to 45.8% among teachers. Improvements were significant for children and parents but not for the teachers. Despite the relatively short intervention period and a relatively non-specific spectrum of interventions, there is some evidence that SN may contribute to strengthening HL within the school setting. The longer-term effects of SN on health literacy and child health should be further examined. For this, a clearer conceptualization of the scope of work of the SN in Germany including their educational interventions is imperative.