School board candidates supported by local teachers' unions overwhelmingly win and we examine the causes and consequences of the "teachers' union premium" in these elections. First, we show that union endorsement information increases voter support. Although the magnitude of this effect varies across ideological and partisan subgroups, an endorsement never hurts a candidate's prospects among any major segment of the electorate. Second, we benchmark the size of the endorsement premium to other well-known determinants of vote-choice in local elections. Perhaps surprisingly, we show the endorsement effect can be as large as the impact of shared partisanship, and substantially larger than the boost from endorsements provided by other stakeholders. Finally, examining real-world endorsement decisions, we find that union support for incumbents hinges on self-interested pecuniary considerations and is unaffected by performance in improving student academic outcomes. The divergence between what endorsements mean and how voters interpret them have troubling normative democratic implications.