The effect of gravity on time and space is an advanced physical topic and cannot be explained in its entirety in a school setting. Nevertheless, it is possible to simulate the gravitational effect with a rather inexpensive model experiment consisting of a cloth and spheres.
With the use of video analysing software like Tracker, the trajectories of the spheres can be highlighted and complex phenomena like formation of planitary orbits or gravity assited maneuvers can be illustrated.
You can Download Videos and Tracker-Data for the following Model-Experiments
(1) Orbit around a central body
(2) Conjoined movement around a central body
(3) Formation of planetary orbits
(4) Gravity assist maneuver