Although best practice for children's mental health services emphasizes ongoing assessment and monitoring of clinical progress, community based clinicians inconsistently implement assessment as part of clinical care due to a variety of practical barriers. The current study explores which factors may be related to the use and function of evidence-based assessment (EBA) in real-world, clinical settings, particularly school mental health (SMH). Mixed methodologies surveying a national sample of SMH clinicians and interviewing clinicians and program managers were used to explore current assessment practices, including use of specific tools and barriers, facilitators and attitudes toward EBA. Results indicate that clinician level of experience is negatively related to overall attitudes toward EBA, particularly openness. The most commonly-reported barriers to using assessments were difficulty reaching parents, respondents not understanding items, and clinicians not having access to measures they like or need. Also, supervision, when received, does not often include EBA. Academic indicators were more regularly collected than any of the 18 clinical assessment tools queried. Qualitative themes including barriers and facilitators to conducting EBA, specific measures' weaknesses and strengths, strategies to increase response rates and regular administration, and program management considerations regarding EBA implementation provide supporting details to these results. Implications for ongoing quality improvement efforts by program managers and clinicians related to the feasible implementation of EBA in school mental health settings are discussed.