This paper explores participation trends in interventions that promote self-evaluation exercises on the effective use of digital technologies in schools. We use a unique dataset consisting of 83,185 respondents from 924 Spanish schools that used SELFIE, a tool based on self-reflection questionnaires that capture different dimensions of school's digital capacity. We benefit from a natural experiment situation caused by the parallel use of SELFIE by two groups of schools. The first group was externally selected as part of a representative sample of Spanish schools. Conversely, the second group voluntarily decided to use SELFIE as a diagnostic tool for a subsequent self-evaluation exercise. Moreover, a subset of schools were located in regions where authorities embedded SELFIE in broader digitalisation programmes. By comparing these groups, it is shown that schools that decide to participate in SELFIE voluntarily are those with a lower initial digitalisation level. It is also found that the promotion of the use of SELFIE as part of public interventions can increase participation but mainly attracts digitally advanced schools. In conclusion, policy interventions aiming to develop the digital capacity of schools need to plan how to reach those schools that need it more in order to be more equitable.