Previous studies have shown that social background inequality differs among educational transitions and it is stronger for those transitions that involve a higher risk of social demotion. This paper focuses on two processes that may account for part of the observed differences in social background inequality across educational transitions. First, it studies how the family of origin might compensate for a ‘false step’ in the early stage of young people's educational careers. This compensatory effect of social background can be described as the likelihood of having ‘a second chance’ for unsuccessful educational transitions. Second, it focuses on two unobserved factors that might potentially bias the effect of social background across educational transitions. These are the students’ unobserved cognitive and non-cognitive skills and their unobserved anticipated choices of dropping out of the education system. Two issues – the compensatory effect of social background and selection bias in educational transitions – are addressed by estimating a probit model with sample selection for the transition to post-compulsory education in Spain.