There's plenty of material which can be adapted for students aged 11-14. For example, activity A3 describes how students can investigate whether launch angle affects the range of a rocket. Students could look at how increasing the number of launches at each angle affects the pattern of results for each launcher. A bottle rocket launcher needs a lot of launches to establish a clear pattern, while a compressed air launcher gives much more consistent results.
In activity A4, Students can compare the motion of two different kinds of rocket as they are launched – the compressed-air rocket used previously, and a water rocket. Using video of the rockets launched vertically, students can the use measurements from each video to compare the distances moved between frames and to describe the motion of each rocket qualitatively. They should notice that the water-powered rocket shows a different pattern of acceleration from the compressed-air rocket. The driving force on the water rocket (produced by expelling water under pressure) lasts longer than for the compressed-air rocket, so it continues to accelerate after it has left the launch pad. Students could go on to calculate velocities using these measurements.
Another method of analysing the flight is shown below.