The headline
Among US brick-and-mortar public schools with at least 500 students, 1,278 reported student-teacher ratios above 25:1 in the most recent NCES data, and 194 reported ratios above 30:1. These are not virtual or self-paced schools, where the metric works differently. These are schools with physical classrooms and full-time teachers.
Where overcrowding clusters
High student-teacher ratios concentrate in a handful of geographies. Fast-growing Sun Belt districts where teacher hiring hasn't kept pace with enrollment growth (DFW exurbs, Phoenix, Tampa). Urban districts that have lost teaching staff to the broader teacher-shortage crisis. And some rural districts where a single departing teacher pushes the ratio dramatically.
What the ratio actually measures
Student-teacher ratio is not the same as average class size. It includes specialists, librarians, counselors with teaching credentials, and resource teachers. So a 20:1 ratio typically corresponds to homeroom classes of 25-30 students. A 25:1 ratio implies homeroom classes well above 30. A 30:1 ratio implies classes that exceed state thresholds in most states with class-size laws.
Where the research is clearest
The research on class size's effect on outcomes is contested in the aggregate. There is broad agreement on the lower-grade tail: classes of 28+ students in K-3 measurably depress reading proficiency, especially for low-income students. The schools in this dataset that combine 25:1+ ratios with elementary grades warrant the closest attention.
Methodology
Source: NCES Common Core of Data 2024-25. Filter excludes virtual schools (those delivering all or most instruction online), schools with fewer than 500 students (where ratio is noisy), and schools where the teacher count is reported as zero or missing (data-quality artifact). The student-teacher ratio is computed by NCES as enrollment / FTE teachers.