This paper analyzes the impact of the allocation of students in different school shifts on levels of segregation in the Rio de Janeiro municipal public schools. The policy is observed in the majority of cities in Brazil and other developing countries, and basically organizes pupils into "school shifts", that is, morning and afternoon sessions. The research design captures the "net effect" of the policy and provides figures to compare the current segregation levels with a hypothetical scenario with no such policies. There are two possible outcomes for the test: 1) no impact; 2) an increase in segregation. Since there is no specific legislation to regulate the allocation of pupils across shifts, a random allocation would be expected. The question to consider is if a random allocation actually occurs.

The term segregation used here refers to an uneven distribution of pupils with similar characteristics across a school system and assessed utilizing the Segregation Index (referred to here as GS). Four different indicators of potentially disadvantaged pupils were calculated a) poverty; b) parents' education; c) colour1; d) age/grade "distortion" (being left back). This last variable summarizes information on all pupils that have not followed a regular age/grade flow in different educational transitions.

The concept of segregation should not be considered a synonym of discrimination or unfairness. It is possible to state that - as measured here - segregation is almost unavoidable to some extent. Nevertheless, the importance of being aware of the phenomenon in order to discern expected or "fair" segregation from inequality enhancing practices and policies should be highlighted.

Evidence from many different countries suggests that school segregation is a universal phenomenon and has to be considered as a consequence of residential segregation, educational policies and parental choice, which are presumed to correlate with social, economic and cultural isolation (^{HARRIS, 2011}). The paper focuses on the role of just one educational policy and, therefore, a large part of variation in the segregation levels will not be explained by this model. The impact of residential segregation, parental choice and other elements of the educational policy should be addressed in future works in order to have a better understanding the causes of segregation in public school system in Brazil.

Within the international debate about school segregation, two crucial questions stand out. The first one is related to the impact of clustering pupils with similar characteristics. Are there any potential benefits or deleterious effects to intentionally clustering pupils? Evidence from different educational systems suggests quite different effects for segregation. On the one hand, it is reasonable to argue that clustering pupils with specific characteristic/needs can be efficient when seeking to implement focused policies directed to help these groups. On the other hand, there is an increasing amount of evidence suggesting that clustering pupils with similar characteristics can have an impact on how they are treated at school, the quality of teaching, overall levels of achievement, the probability of moving on to higher education; an increasing association between academic achievement and socio-economic status (HAARTH et al., 2005; ^{EGGRES, 2005}; ^{BRITO; COSTA, 2010}; ROSENTHAL; JACOBSON, 1968).

A second question refers to the role of educational policies on segregation levels. Do the policies influence the overall level of segregation? It is reasonable to say that policies that deliberately aim to separate pupils based on, for example, skin colour has lost legitimacy throughout the years. The apartheid system in South Africa, for instance, which intentionally segregated pupils based on ethnicity (white and black population) is most likely to be considered unfair and illegal in most democratic countries.

Nonetheless, the decision to intentionally cluster pupils with similar characteristics can be seen as fair and desirable when seeking to make the educational system less stratified, at least in terms of student achievement.

Differentiation procedures can be applied with the objective of diminishing existing social inequalities. The new understanding of what is fair in terms of educational opportunities creates a new opposition between more comprehensive educational systems and more segmented systems, with different "types" of schools, curriculum and incentives. However, new policies that endeavour to address preexisting social inequalities can also inadvertently increase segregation. This "adverse effect" should be taken seriously by researchers, since it can interfere with the possible benefits of the policy.

There are many examples of policies that can unintentionally impact the segregation levels. Charter schools in the U.S. are one example of an attempt to make the educational system more diverse and appealing. Another example is the tracking system that takes place, for example, in Germany or Hungary. There is robust evidence suggesting that the allocation of pupils in a stratified educational system is highly correlated with pupils' socio-economic status. The attempt to track by pupils' ability can, at least in some cases, end up being not very different from selecting based on socio-economic status. A third and last example is the School Choice policy that, among other things, is intended to increase parental choice. There is no consensus among researchers on the impacts of incentive for choosing schools on school segregation (GORARD; TAYLOR; FITZ, 2003).

All policies mentioned above are intended to increase both the quality and equity of educational systems. Despite the fact that there is empirical evidence that these two goals are not incompatible, it seems a hard equation to resolve. Robust research designs that can estimate, not only the intended impact of educational policies, but also the unintended ones, can help policy makers to take further action to improve the educational system.

The paper is divided into six sections, including this introduction. The next section contextualizes the public school system of Rio de Janeiro,, offering data about the policy of allocating students by shifts Following this is the description of the study design, presenting the Segregation Index and the main variables used to describe students with potential disadvantage. The fourth section describes patterns of segregation in the municipal public school network. The fifth provides results of the impact of shift allocation on school segregation. The sixth and final section highlights the main findings, discussing their potential future use in terms of educational policy.

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