This study examined beginning English literacy-skill development and achievement among Spanish-speaking children enrolled in state-mandated English-only classrooms. The children possessed Spanish skill at or above age-appropriate level, yet minimal English skill, and came from a Spanish-speaking community adjacent to the U.S.-Mexico border. Under its English-only law, the state-mandated classroom instruction is only in English through a structured English immersion model (SEI) using state-regulated and supervised four-hour English language development blocks for these children. Using secondary analysis of existing data, children's English vocabulary, phonemic awareness, word-reading fluency scores, as well as their English reading and language achievement scores were examined for four years with two separate cohorts of children, beginning with their entry into kindergarten and ending at third grade (K-3rd). Analyses indicated that English-only SEI instruction for children did not result in age- or grade-level-appropriate development or achievement for all children. In fact, less than one-half met age- or grade-level performance.