The new school class-size law: estimating its economic impact

In Spain, the maximum number of pupils per classroom is 25 in Primary education and 30 in lower secondary education (ESO). On 11 November 2025, the Ministry of Education, Vocational Training and Sport (MEFPD) presented the draft Bill to reduce the maximum class size to 22 pupils in Primary and 25 in ESO, and to introduce a double count of pupils with special educational needs (SEN) in mainstream classrooms.

Schools whose class sizes exceed the new thresholds will have to open new classrooms in the relevant year groups. Opening new groups will raise spending, as schools will need to hire additional teachers and build these new classrooms. This additional spending is what we may call the “real cost”, that is, what many schools will have to spend over and above current levels as a result of the reform. At the same time, Spain is losing its child population because successive birth cohorts are becoming smaller. Demographic projections expect this widespread decline in the number of students to accelerate even further in the coming decades, which will mean that the new class sizes can be accommodated naturally, fully or partially, within a less populated system. This would constitute a “demographic dividend”: the savings produced by freeing up part of schools’ staff as a result of the natural decline in the number of pupils. The “net budgetary cost”, or what the public administration will have to add to its fiscal expenditure as a result of the reform, is calculated as the difference between the real cost and the demographic dividend. Estimating all of these values precisely (the demographic dividend and the budgetary cost, and therefore their sum, the real cost) is of vital importance for Spanish public education policy, since it will determine whether the reform is cost-effective.

This Policy Brief uses data from the MEFPD, INE, PISA, TIMSS and PIRLS to quantify, in the most conservative and precise way possible, the real cost, the demographic dividend and the budgetary cost over the coming decade. To help contextualise and offer an answer regarding the potential cost-effectiveness of the reform, it develops a review of the empirical evidence on the effect of reducing class sizes, allowing the expected impact on learning and teacher well-being to be gauged, alongside a comparison with the alternative measures to which the resources that the reform would displace could be allocated.

The key findings of this analysis are the following:

• Over the coming decade (2027-2036) the average net budgetary cost will be at least €2.818 billion per year, with a cumulative total of €28.184 billion. The peak year of the net budgetary cost is equivalent to 13.8% of Spanish public spending on Primary education and ESO.
• In the peak year of the reform (2031), that is, when the reform is fully implemented in both Primary and ESO: (i) the real cost will be €6.549 billion; (ii) the demographic dividend will be €1.380 billion; (iii) the net budgetary cost will therefore be at least €5.169 billion.
• Andalusia, Catalonia and Madrid concentrate 63% of the national cost.
• The reform will channel more resources to schools with a higher share of students of higher socioeconomic status. Schools with the highest share of higher-socioeconomic-status pupils will concentrate 31% of the administration’s spending, compared with 22% for those of lower socioeconomic status. This is because the classrooms that exceed the new ceiling are more concentrated in urban areas with higher demand and, therefore, with higher socioeconomic profiles.
• The most comprehensive and recent academic evidence (the meta-analysis by Opatrny et al., 2025, covering 66 causal studies) indicates that reducing class size has no significant aggregate effects on student learning, the grade-repetition rate or well-being with regard to school and classmates. It does show measurable but heterogeneous effects on the well-being of families and teachers. Specifically, reducing class size improves teachers’ satisfaction with their work, but the effect is modest: expressed in terms of an equivalent pay rise, that gain in well-being would fall below the salary increase that the same budget would finance if it were allocated directly to teachers’ salaries (6% vs 21%). This policy therefore entails a very high cost relative to the effects it can produce according to the evidence gathered.

In line with the results obtained, this Policy Brief suggests three recommendations:

• Limit the universal rollout of the measure solely to schools of high complexity and educational risk, where the evidence suggests somewhat larger effects and, more generally, where educational investments have the greatest impact according to the evidence. This application should be carried out with piloting and constant evaluation of its effects, and should be accompanied analogously by complementary policies that amplify its effects.
• Redirect part of the budgetary effort towards policies with more robust evidence of impact, such as PROA+-style intensive tutoring, salary supplements for teachers posted to high-complexity schools, and an educational “MIR” (residency-style induction programme).
• Make use of the demographic window that opens over the next ten years not as budgetary room dedicated solely to the universal reduction of class sizes, but as a lever to invest where the evidence indicates that the educational return is greatest, that is, especially among students of lower socioeconomic status.