The Colorado Department of Education evaluated the 18 federally funded tutoring organizations* that served children in grades k-3 in the 2009-2010 school year. This CDE report included an analysis of the percentage of students reading below grade level that were moved up to on or above grade level. The results are plotted below.
* These organizations are called Supplemental Educational Services Providers and are funded by Federal dollars given to the states under No Child Left Behind.
Specically, these SES Providers provide tutoring for public school students who are identified as low income and attending a low performing school.
NW Coalition for Better Schools Gives Best Reading Gains in Colorado
Percentage of SES and Comparison Students with DRA2 Scores who Improved from 2009 to 2010 in Grade Level Target by Vendor (2009-10).
"OMNI Evaluation of NCLB Title I, Part A: Supplemental Education Services, Evaluation Three Year Report. Submitted to the Colorado Department of Education, June 2011.
The graph below presents the reading gains from above as a blue line, and then the cost per student as red bars. The point to note is that in addition to having the highest reading gain our organization also has the lowest cost per student.

This is all the more significant since we are the only ones using older children as tutors, and those tutors have a ninth grade dropout rate that was one sixth that of the Denver Public School ninth grade average dropout rate. That makes this tutoring model effective for improving literacy and reducing student dropout rate.
Being a full year tutor raises graduation rate to 91%!
Serving as a tutor has been found to raise graduation rates of students to 98% in Texas in the Coca-Cola Valued Youth Program, (CCVYP) but we needed to know if our program achieved similar results.

Our long-term study on tutor graduation rates.
To answer that question, we examined the graduation rates of students who had been tutors with us anywere in the past 8 years. We had 177 tutors who should have graduated by now. We were also able to obtain information on 150 students who were matched by Denver Public Schools to serve as a control group. The data for both groups of students are presented below, first as a table, and then as a graph.

Control research paper Group
The control group of students had a 64% graduation rate. Source.

Examination of magnitude of self-selection bias
Because the students who agree to tutor might already be on a better track to graduation, we calculated the graduation rate of those students who tutored only a small number of hours. We combined the students who tutored 1 to 15 hours with those who tutored 15 to 30 hours to create a larger group to examine.
The students who tutored with us for only 1 to 30 hours had an
average graduation rate of 77%.

Effect of being a tutor on the student's graduation rate
The students who tutored 31 to 45 hours had an increased graduation rate, and those who tutored 46 to 60 hours approached the 98% graduation rate found in CCVYP with at-risk students in San Antonio, Texas. We combined the two groups into a 31 to 60 hours group.
The students who tutored tutored for 31 or more hours had an
average graduation rate of 91%.

Multi-year tutors
Our normal tutoring program runs for 50 hours in addition to training time, so the 31 to 45 hours group contained mostly single year tutors while the 46 to 60 hours group contained more students who tutored with us for at least a second year.
This graduation rate and dropout rate are both presented on this praph for completness reasons. The dashed lines are a computer generated best fit line for both graduation and dropout rates. The increased graduation rate with more hours served as a tutor suggests that being a tutor changes some aspect of the student that contributes to persistance to graduation.

Improved patience
In support of the argument that tutoring changes the tutors, we have interview a number of tutors at the end of the year to see if they had noticed any changes in themselves. They all said that they had noticed that their patience had improved, both with the children they were tutoring, and also with family, friends and teachers.

Recognition by others of helping nature
Several other tutors have mentioned that they are noticing classmates coming to them for help now, whereas that hadn't happened before.

These last two observations indicate that being a tutor gives these students life skills that may help them the rest of their lives.