OK, Civil War era shells can be dangerous, in theory. In practice, when was the last time you heard of one actually exploding? During World War Two, Fort Macon in North Carolina was reactivated and the U.S. Army actively manned the fort with Coast Artillery troops.
Just after arriving at Fort Macon and setting up quarters in the fort, a fire was built by some of the men in the fireplace of one of the rooms. However, someone found a couple of old Civil War cannonballs, which had been recovered around the fort, and unthinkingly placed them in the fireplace to serve as andirons. One of the cannonballs was a live shell, which quickly exploded in the fire In a room full of soldiers. Pvt. George Eastep remembered the blast went over him as he lay on his cot,but caught his bedding on fire. Shrapnel rattled against the opposite wall. One man was blown through a doorway into the adjoining room. By some miracle no one was killed. A couple of men had minor injuries but Pvt. Harry Chait had burns that required him to be hospitalized briefly. The entire incident, which was later mentioned in Ripley's "Believe It or Not" newspaper column has been remembered ever since as the "last shot of the Civil War," because the 244th Coast Artillery originally was the Ninth New York National Guard and its men were "Northerners." That included Pvt. Chait, injured by an old Confederate cannonball.
The Ripley's "Believe It or Not" cartoon headline was:
CONFEDERATE SHELL WOUNDS YANKEE SOLDIERS.......... 80 YEARS AFTER IT WAS FIRED
I have a Civil War Bormann shell, Parrott shell and a Hotchkiss shell myself as historical relics but they have all been deactivated.
in general carbon (from charcoal), and sulphur are not soluable. Nitroglycerin and ammonium nitrate (saltpeter) are soluable. Nitrates are the unstable parts.
If it was really heavy, it was a solid. If it was light, then it was a hollow with charge, and could be explosive.
Course if they didn't have a fireplace in the classroom, there wasn't much reason to fear.