Found this research via Freakonomics. There has been quite a lot of research lately on the importance of school size. Hattie (2009) describes in Visible Learning that optimal school size lies between 600 and 800 pupils. The reasons for this minimum and maximum is more important. If a school is too small, there are too few possibilities for professional development, too big can make a school too anonymous. 

From a press release earlier this year on earlier research of even smaller high schools in New York. The interesting bit is that the enrollment in these schools is lottery-based, bypassing the bias that makes this kind of research sometimes very difficult:

• Small high schools in New York City continue to markedly increase high school graduation rates for large numbers of disadvantaged students of color, even as graduation rates are rising at the schools with which SSCs are compared. For the full sample, students at small high schools have a graduation rate of 70.4 percent, compared with 60.9 percent for comparable students at other New York City high schools.
• The best evidence that exists indicates that small high schools may increase graduation rates for two new subgroups for which findings were not previously available: special education students and English language learners. However, given the still-limited sample sizes for these subgroups, the evidence will not be definitive until more student cohorts can be added to the analysis. 
• Principals and teachers at the 25 small high schools with the strongest evidence of effectiveness strongly believe that academic rigor and personal relationships with students contribute to the effectiveness of their schools. They also believe that these attributes derive from their schools’ small organizational structures and from their committed, knowledgeable, hardworking, and adaptable teachers. 

New research I found via Freakonomics investigates the same effect in the same schools and the abstract makes it very clear (download the research here):

One of the most wide-ranging reforms in public education in the last decade has been the reorganization of large comprehensive high schools into small schools with roughly 100 students per grade.  We use assignment lotteries embedded in New York City’s high school match to estimate the effects of attendance at a new small high school on student achievement. More than 150 unselective small high schools created between 2002 and 2008 have enhanced autonomy, but operate within-district with traditional public school teachers, principals, and collectively-bargained work rules.  Lottery estimates show positive score gains in Mathematics, English, Science, and History, more credit accumulation, and higher graduation rates.  Small school attendance causes a substantial increase in college enrollment, with a marked shift to CUNY institutions.  Students are also less likely to require remediation in reading and writing when at college.  Detailed school surveys indicate that students at small schools are more engaged and closely monitored, despite fewer course offerings and activities.  Teachers report greater feedback, increased safety, and improved collaboration.  The results show that school size is an important factor in education production and highlight the potential for within-district reform strategies to substantially improve student achievement. (bold by Freakonomics)