In school dental screening, a dental health professional visually inspects children's oral cavities in a school setting and provides information for parents on their child's current oral health status and treatment needs. Screening at school aims to identify potential problems before symptomatic disease presentation, hence prompting preventive and therapeutic oral health care for the children. This review evaluates the effectiveness of school dental screening for improving oral health status. It is the second update of a review originally published in December 2017 and first updated in August 2019. To assess the effectiveness of school dental screening programmes on overall oral health status and use of dental services. An information specialist searched four bibliographic databases up to 15 October 2021 and used additional search methods to identify published, unpublished and ongoing studies. We included randomised controlled trials (RCTs; cluster- or individually randomised) that evaluated school dental screening compared with no intervention, or that compared two different types of screening. We used standard methodological procedures expected by Cochrane. The previous version of this review included seven RCTs, and our updated search identified one additional trial. Therefore, this update included eight trials (six cluster-RCTs) with 21,290 children aged 4 to 15 years. Four trials were conducted in the UK, two in India, one in the USA and one in Saudi Arabia. We rated two trials at low risk of bias, three at high risk of bias and three at unclear risk of bias. No trials had long-term follow-up to ascertain the lasting effects of school dental screening. The trials assessed outcomes at 3 to 11 months of follow-up. No trials reported the proportion of children with treated or untreated oral diseases other than caries. Neither did they report on cost-effectiveness or adverse events. Four trials evaluated traditional screening versus no screening. We performed a meta-analysis for the outcome 'dental attendance' and found an inconclusive result with high heterogeneity. The heterogeneity was partly due to study design (three cluster-RCTs and one individually randomised trial). Due to this inconsistency, and unclear risk of bias, we downgraded the evidence to very low certainty, and we are unable to draw conclusions about this comparison. Two cluster-RCTs (both four-arm trials) evaluated criteria-based screening versus no screening, suggesting a possible small benefit (pooled risk ratio (RR) 1.07, 95% confidence interval (CI) 0.99 to 1.16; low-certainty evidence). There was no evidence of a difference when comparing criteria-based screening to traditional screening (RR 1.01, 95% CI 0.94 to 1.08; very low-certainty evidence). One trial compared a specific (personalised) referral letter to a non-specific letter. Results favoured the specific referral letter for increasing attendance at general dentist services (RR 1.39, 95% CI 1.09 to 1.77; very low-certainty evidence) and attendance at specialist orthodontist services (RR 1.90, 95% CI 1.18 to 3.06; very low-certainty evidence). One trial compared screening supplemented with motivation to screening alone. Dental attendance was more likely after screening supplemented with motivation (RR 3.08, 95% CI 2.57 to 3.71; very low-certainty evidence). One trial compared referral to a specific dental treatment facility with advice to attend a dentist. There was no evidence of a difference in dental attendance between these two referrals (RR 0.91, 95% CI 0.34 to 2.47; very low-certainty evidence). Only one trial reported the proportion of children with treated dental caries. This trial evaluated a post-screening referral letter based on the common-sense model of self-regulation (a theoretical framework that explains how people understand and respond to threats to their health), with or without a dental information guide, compared to a standard referral letter. The findings were inconclusive. Due to high risk of bias, indirectness and imprecision, we assessed the evidence as very low certainty. The evidence is insufficient to draw conclusions about whether there is a role for school dental screening in improving dental attendance. We are uncertain whether traditional screening is better than no screening (very low-certainty evidence). Criteria-based screening may improve dental attendance when compared to no screening (low-certainty evidence). However, when compared to traditional screening, there is no evidence of a difference in dental attendance (very low-certainty evidence). For children requiring treatment, personalised or specific referral letters may improve dental attendance when compared to non-specific referral letters (very low-certainty evidence). Screening supplemented with motivation (oral health education and offer of free treatment) may improve dental attendance in comparison to screening alone (very low-certainty evidence). We are uncertain whether a referral letter based on the 'common-sense model of self-regulation' is better than a standard referral letter (very low-certainty evidence) or whether specific referral to a dental treatment facility is better than a generic advice letter to visit the dentist (very low-certainty evidence). The trials included in this review evaluated effects of school dental screening in the short term. None of them evaluated its effectiveness for improving oral health or addressed possible adverse effects or costs.