The headline
"Overcrowded" has no federal definition. Most state laws set class-size caps in K-3 between 20 and 25 students; some set caps in 4-12 between 25 and 32. Schools whose reported student-teacher ratios approach or exceed these thresholds are running overcrowded classrooms in practice.
Based on NCES 2024-25 data, roughly 1,278 brick-and-mortar US public schools with 500+ students report student-teacher ratios above 25:1. The actual homeroom class sizes in these schools are typically larger than the ratio implies, because student-teacher ratio counts specialists and coaches as teaching staff.
Where overcrowding clusters
Three patterns dominate:
- Sun Belt growth districts where teacher hiring hasn't kept pace with enrollment surges. Several Phoenix-area, DFW-area, and Florida districts feature prominently.
- Teacher-shortage hotspots. Districts that have struggled to fill teacher vacancies, particularly in math, science, special education, and English-learner support, operate with elevated ratios as an emergency measure rather than by design.
- Urban districts with consolidated buildings. When a district closes one school and routes students to a neighboring building, the receiving school's ratio often spikes for the year or two it takes the staffing to catch up.
What state law says
About 30 states have legal class-size caps for at least some grades. Enforcement varies dramatically. Some states tie funding to compliance; others treat the caps as aspirational. The schools in this dataset whose ratios indicate likely violations of their state's caps are flagged.
Where class size actually matters
Class-size research is more nuanced than activists or skeptics often portray. The strongest evidence is for K-3, where classes above 25-27 students measurably depress reading proficiency, particularly for low-income students. Above 4th grade, the effects are smaller. The schools combining 25+ ratios with elementary grades warrant the closest scrutiny.
Methodology
Source: NCES Common Core of Data 2024-25. Filter excludes virtual schools, schools under 500 students (small-N noise), and schools where the FTE-teacher count is missing or zero (data artifact). Ratio computed as enrollment / FTE teachers.