Distribute rubberbands (big fat ones are the best) and rulers to students. Caution them not to stretch their rubberbands unnecessarily (since this will change their elasticity and thus change the calibration). Place a tape measure on the floor (or ground outside), extended to about 20 feet should be sufficient.
Have each student shoot his/her rubberband four times, using their ruler like a gun, and measure the distances. Each time the rubberband should be pulled back a little further, e.g., 6", 8", 10" and 12". The stuidents need to hold their rulers waist high and level each time. This is important for accurate calibration. The results of their shootings are recorded then curve produced, plotting distance of flight on the Y axis versus distance pulled back on the ruler (stretch) on the X axis.
Once each student has his/her calibration curve completed take the class outside to the playground. Using parallel lines painted on the blacktop for four square, basketball, etc., measure the distance from one line to another (making sure that it is within the calibration range of the student's rubberbands). Line the students up on one line, give them the measured distance, instruct them to use their calibration curves to determine the amount of stretch they will need to make their rubberbands go exactly the measured distance, then shoot.
My experience is that most students will hit within 6 to 12 inches of a target 15 feet away. As a bonus, prizes can be awarded for hitting the target, being within 6 inches, etc.