Those who think the Holy Grail has been found in Tenth Grade obviously do not watch Antique Road Show. The makers mark on the bottom of the clay cup is that of Sid Yahweh, a Mesopotamian maker of clay cups which were sold in the market filled with peanut butter, then later used as drinking cups.
They have very little value. His later works where he decorated them with popular cuneiform carton characters, especially the Peter Pan series are of some value but there were so many made they can still be found in roadside stands.
Why the cup in question was buried in a special box we will probably never know. I suspect the author likes to play with people minds on occasion.