Opaque white glue contains polyvinyl acetate. The MSDS says that it is a "PVAC BASED ADHESIVE". Look at this page at Sci-Toys:
Vinyl acetate. No idea what else may be in it, in addition to the PVA. One site says propylene glycol. The formula is not acid-free, so people working in museums are advised against its use. Carol Soderlund, who has promoted the use of the washable blue gel as a resist in her classes, says not to use white glue as a resist. Not sure whether this is just because it does not work as well for her methods, or if it's because of the discharge effect that you observed.
The original formulation of white glue was a milk-based adhesive containing the protein casein, but the formula was changed somewhere around 1980. (
source.) Other brands of white glues based on casein are probably still around.
Clear blue washable school gel also is based on polyvinyl acetate. The MSDS describes it as a "PVA BASED ADHESIVE". What makes it washable? Possibly the presence of some sort of detergent. Again, no idea of other ingredients.
Paula