To do this experiment you will need some materials to measure shadows accurately. Your "gnomon" will be a meter stick that is perpendicular to the ground. For your measurements to be accurate, it is critical that the meter stick be vertical. (Note the devices used below. Wind can be a major factor. Just ask Kathleen Smith. She held a contest to see which group could come up with the sturdiest measurement station.) 
Fort Worth, Texas 
Manasquan High School Manasquan, New Jersey 

Site  Location on globe 
latitude 
Location on globe 
longitude 
Shadow length  Sun angle 
(Your school's name goes here)  .  .  .  . 

You might want to make a chart and place it next to a wall map of the world. Use the map to mark where the participating schools are located. 
Chart 2
Site  Location on globe 
latitude 
Location on globe 
longitude 
Shadow length  Sun angle 
your school  .  .  .  . 
school #1  .  .  .  . 
school #2  .  .  .  . 
Etc.  .  .  .  . 
After you have several schools with entries, the students pick out one of the schools to complete the chart with. Have your students extend the chart to include the central angle, circumference, and percentage error.
Chart 3
Site  Location on globe 
latitude 
Location on globe 
longitude 
Shadow length  Sun angle  "Center of the earth" angle  N/S Distance  Circum
ference 
% error 
your school  .  .  .  .  .  .  .  . 
school #2  .  .  .  .  .  .  .  . 
School #3  .  .  .  .  .  .  .  . 
Etc.  .  .  .  .  .  .  .  . 
Now that you know the central angle, draw
the location of your school and another school on the circumference of
a large circle.
Additional notes:
The distance from your school to another site is not the "as the crow flies" distance, but rather the north/south distance. As a result this can be more of a challenge. Have students use map scales to determine the distance. The least satisfactory, but very accurate method is to determine the north/south distance from the site's latitude. Though the math is good here, the need for using the actual circumference of the earth as part of the calculation is a bit of deception since you are using information that you are setting out to show. What the ancients did was hire surveyors or behamists to measure the distances needed. A modern solution would be to drive from one site to another and use the speedometer as your measuring device. Of course, the sites would have to be on the same longitudinal line. A compromise would be to have the students use scale methods for estimating the distance, then comparing them to the more accurate methods to see how they did. The compromise would be to choose the more accurate measurement for the purposes of this activity.