BACKGROUND: School servants are professionals involved with cleaning and feeding activities in schools. Exposed to conditions of physical and psychosocial overload at work, they may face situations of morbidity and functional restrictions. OBJECTIVE: To evaluate personal and work characteristics and work ability index (WAI) of school servants in Brazil and test the mediating effect of WAI in the duration of sickness absence. METHODS: 163 Brazilian school servants participated in this cross-sectional study, that including the self-completion of a sociodemographic questionnaire, information about the Work Environment, the Protocol of Psychosocial Risks at Work’s Evaluation, and the Work Ability Index. Also was collected, in a secondary health database, information about sickness absence. A linear regression model was constructed to identify the predictors of the duration of absence, followed by the analysis of mediation with the Path Analysis method. RESULTS: The study found that the WAI partially mediates the relation of the personal characteristics and the work conditions with the duration of absence of the school servants. The predictors included the organizational aspects of work, mental exhaustion, physical damage, health self-evaluation, availability of personal protective equipment, and the need to assume inadequate postures at work. These predictors explained 33% of the duration of absence and 53% of the WAI of these workers. CONCLUSION: This study indicated that inadequate working conditions and low WAI impact in the sickness absence, increasing by 33% the length of time off work of school servants.