The graph on figure 2 of Collier and Thomas' article (p. 8) also attests to the efficacy of two way immersion classes, perhaps even more so than the first graph. This is primarily due to the fact that there is a greater disparity in the average test scores for students who were enrolled in these classes, versus those that are not. Furthermore, since the learning of English is the principle objective for dual language learners, this graph attests to the value in students enrolling in two way immersion courses in which they learn both Spanish and English.

Another fact that this second graph alludes to, and which is noted in comparison efforts with the first graph, is that there is a relationship between achievement in Spanish and achievement in English. This is particularly true for non-native English speakers. Learning basic fundamental aspects of their own language naturally correlates to a greater understanding of these fundamental concepts in a foreign language. All too often, non-native...

These graphs suggest that by teaching fundamentals in one language, students can better master those same concepts in another.
In conclusion, one of the most effective means of closing the achievement gap between native English speakers and non-natives is to implement two-way immersion classes. The principle distinction between a two-way class and a one way class is that the former allows for native English speakers to enroll in them. This allows for a degree of student expertise in English even at the youngest grade levels. These students inevitably help the others, in much the same way that the other students help these native English speakers master a non-native language (such as Spanish).

References

Collier, V.P, Thomas, W.P. (2004). "The astounding effectiveness of dual education for all." NABE Journal of Research and Practice. 2:1.