Louie Villalobos summarizes a recent study by the Arizona Department of Education:

The Arizona results showed students in immersion classes outperformed bilingual education students in every grade level between second and eighth grade in reading, language and math, based on Stanford 9 scores.
There starts to be a significant difference at the sixth-grade level, at which immersion students were more than one year ahead of the bilingual students in math.
By the eighth grade, there was at least a one-year difference in all three subjects.
“There is not a single exception,” Horne said. “It tells us that the students in English immersion do substantially better.”