On hand, the APPRENTICE has sticks of chalk (which can be powdered with the mortar and pestle later), and a vial of BRIMSTONE. If they want to make green glow sticks, grass might work for the COLOUR REAGENT.

It's raining outside, so they could get water that way, albeit it'd be slow to fill the cauldron that way.

Thankfully, the APPRENTICE knows where Whisps are: the "lights" in the foyer. The difficulty will be extracting the ESSENCE of them, somehow.