There are two types of stain, one from over heating the metal and one from solder flow. Firestain comes for oxidizing the copper below the surface level of the sterling silver and the other is where the path of solder flow is visible on the surface of the silver and the remedy for both is mechanical removal. In other words, abrasion or sanding, filing or grinding and then re-polishing the surface. To cut down on firestain you need to have your students watch getting the metal to hot and for the solder stain the use of a solder pick to place solder carefully will help with that.