Q:

Desperately Need Help with Grade 9 Static Electricity

I am preparing to teach Grade 9 Static Electricity next week and am going crazy trying to figure out what is happening in one of my experiments.  I have a short piece of PVC pipe, 4 inches diameter, and I rub it with wool to charge it negatively.  I can observe excellent repulsion when I touch it with my foil bit (dangling from a thread).  Here is the problem: I am holding down the PVC pipe down on a wooden base with two brass-plated wood screws, and these screws somehow collect an INSANE amount of positive charge, even when I am very careful not to touch them with the wool.  The foil bit is strongly  attracted to the screws, and when it touches them it bounces off more violently than anything I've seen in any of my other static electricity experiments. 
Can anybody explain what I am seeing? 

A:

I would guess that the screws are secretly grounded through a hidden conducting path inside the wood block and the table.  Maybe the table is wet, maybe there are metal wires inside the block, I don't know the exact setup, but I think there is some grounding path.  This means that they stay at zero potential, no matter what you do to the PVC.  So when you charge up the PVC tube, the screws accumulate a large amount of charge to keep their potential zero in the tube background. 
The induced charge on grounded screws is much larger than the induced polarization on isolated screws, and the pointiness of the screw will lead to huge local fields near them, and will explain your observation.  If this is the case, putting big books beneath the block should de-ground the screws. 
For this explanation to work, the screws don't have to be completely grounded.  It is enough if you have the screws go all the way through the wood, and rest the block so that the screws touch a metal table.  In this case, the metal will act as a not-so-infinite ground, and will supply charge to the semi-grounded screws.  You can fix this by mounting the block on top of a dry book. 
To test any of these explanations, you can just hold up the block yourself without touching the screws, and rub, and see if you still get the large charge on the screw.