Briefly:TCE is not "a chemical that is known to sink" In large scale spills it tends to remain in pockets just over the bedrock layer.
Research the term: Dense non-aqueous phase liquid
At an average of 50 to 100 thousand gallons for a small fracturing well, it is often convenient to drill a water well nearby, or to buy water from a nearby well. I have no idea what the water source here was.
Response to poisons differ with means of ingestion.
I have no doubt that experts, perhaps including some from Paragon Environmental Construction, have insured the populace that everything is all right.
There are many possible sources of chemical pollutants near the school. Those that can cause nerve damage may leave victims more susceptible to other disease vectors.
Workers are often better protected than the people who live with their waste products.
Aerosol testing and a positive attitude are not enough; get some scientists in there; move the school; don't take chances with your children's lives.