School boards remain one of the most powerful forces in American education, helping to set curricula, evaluate teachers, and direct hundreds of billions of dollars in education funding. Yet teachers' unions play an outsized role in determining who serves on these boards. If the interests of teachers are perfectly aligned with those of students, then there may be no reason to worry about union dominance in school board elections. When these interests collide, union power likely encourages boards to prioritize the needs of adult employees over students. This report analyzes the nature of union power in school board elections, and in particular, how unions decide which candidates to support. Key findings include: (1) Union electioneering success is not simply a product of union mobilization. Rather, union endorsements increase voters' support for union-backed candidates by 6 percentage points; (2) The union seal of approval buoys candidates' electoral prospects because voters believe union-favored candidates hold shared interests on important education issues; (3) Voters are largely mistaken about what union endorsements convey and what drives endorsement decisions. The only consistent predictor of union support for incumbents is whether the district raised salaries for senior teachers prior to an election; and (4) The divergence between what union endorsements mean and how voters interpret them have troubling implications for democratic accountability and board-based governance. Groups wishing to counteract union dominance will need to find ways to ensure that ordinary voters are aware of the actual policy priorities of union-backed candidates.