On the stage in the senior hall you should find an iPhone in the drawer under the PC. This is running an app which will let you control the volume. If it doesn't look like the screen below, select QU-16 or QU-PAC and press Connect.
The controls are:
PC: Computer volume
LV1: Lapel style radio mic 1
LV2: Lapel style radio mic 2
LV3: Lapel style radio mic 3
HH4: Hand-held radio mic 4
Press on one of the above items and you will see a fader where you can slide the volume up or down:
For more advanced control, including of the various other inputs, you will need the iPad running QU-Pad, this is locked in the cabinet and should only be used by Tech Services, the Music dept or anyone suitably experienced.