This paper investigates the effects on achievement, study behaviours and attitudes of an intervention providing extra instruction time in language and in mathematics in lower secondary schools in Southern Italy. We use a difference-in-differences strategy and compare two contiguous cohorts of students enrolled in the same class for two consecutive years. We find that an average increase of in instruction time leads to an increase in 0.12 sd in mathematics test score for both females and males, while no effect is found on Italian language test scores. Cross-disciplinary effects seem to suggest that extra-classes in mathematics are beneficial for girls also for language scores. The pattern of results found on attitudes and self-reported study behaviours suggests that girls use the extra instruction time as a complement to regular home study, while boys may use it as a substitute.