School nurses have reported stress and worry caring for young people experiencing mental health problems, but why this occurs and how they respond has not been well-explored. In this study researchers generated a substantive theory of the experiences of secondary school nurses who encountered young people with mental health problems using the original method of grounded theory. Thirty-one Western Australian school nurse participants reported that students presented with complex mental and social health needs that were not easily resolved. This was conceptualized as an "untenable burden". Participants countered this by engaging in the three-stage process of "tactical prioritizing". During an initial period of "strategic assimilation" into the school and broader community, participants referred students to external service providers. This was frequently inadequate, and participants found themselves grappling with unmet student mental health needs. Participants responded by implementing nursing strategies for "optimizing outcomes" in the lives of young people, while simultaneously engaging in "managing self" to support their own well-being.