High Voltage Capacitor
This is a well-known piece of equipment called, variously, a shorting stick or ground stick. You can buy them (although they're pricey) or you can make your own. 

To make your own, take a 2-foot length of wooden stick, say a 1x2. To the end, fix an old multimeter probe. Connect the probe to a string of high-value resistors, say 100k. Look up the voltage rating of the resistors, and use at least as many as this implies for your working voltage. At the end of the string connect a high-voltage wire (rated for your voltage) connected to an alligator clip.

To use, fasten the clip to a handy ground point, and then touch the probe tip to the capacitor terminal. Hold for several seconds. Make sure to check the resistor string value each time you use the stick. If one of the resistors has an internal crack or a solder joint has opened up, the stick won't work and you won't know it.

Once the cap is discharged, put some sort of shorting link on it. Capacitor dielectrics will recover voltage after being discharged. 

Since you're dealing with schoolchildren, you might seriously consider a commercial product. Think, "liability".