This study aims to understand how Arizona's current approach to ESL education \u2014 a recently developed version of Structured English Immersion (SEI) known as the four-hour ELD block \u2014 affects high school teachers and students in mainstream math and science classes containing a mixed population of English Language Learners (ELLs) and native English-speaking students. This focus was chosen due to a lack of prior research on the ELD block's effects at the high school level, and the unique consequences of the ELD block for the timing of ELL students' math and science enrollment. Four teachers of mixed-population, mainstream math and science classes, from four different high schools within a single Arizona district, were interviewed and observed in order to understand their perspectives on their own experiences and those of their students. Areas of focus included students' academic and social experiences, challenges faced by teachers and their responses to these challenges, and teachers' evaluations of their teaching situation and of the ELD block itself. Data were analyzed using modified analytic induction. The study found that teachers believe the following: that the ELD block causes ELLs to be socially isolated; that it damages ELLs' content development; and that, by forcing some ELLs to take mainstream math and science classes before they have received any ESL instruction, it damages their ability to succeed in these classes. Additionally, teachers indicated that they struggled to meet the needs of their ELL students and non-ELL students at once. Given these findings, the removal of the ELD block, and the addition of bilingual, ESL specialist aides into mainstream classes, is recommended as a solution.