Bring a shortwave receiver to school and let students eavesdrop on Amateur Radio retransmissions. Then set up a 2-meter satellite ground station in class, and try a satellite contact. The teacher can apply for a future school contact, almost guaranteeing that students will have an opportunity to communicate with a crew. The ISS crew has not lost sight of why it has been so successful. It is the Amateur Radio community that has brought astronauts voices into schools.
Crew members make random contacts with earth-bound hams.
They make contacts during their breaks, pre-sleep time and before and after mealtime. Astronauts have contacted thousands of hams around the world. Computer software allows the crew to operate the 2-meter packet gear radio in unattended mode, and hams can make contacts when the crewmembers are working. But successful contacts have even been made with verticals and ground plane antennas. Commercial and public domain software is available to help track when a shuttle or the ISS will be in range of your station, and where to point your antenna.