Let harden before covering with dirt. Use flat slab or bricks. Make a plaster slab about one inch thick in a cardboard box. Mix plaster with potting soil and sand to give it an aged look. Student will have to translate the tablets in post-dig activities.
Retail stores carry beads and jewelry supplies in their craft sections.
For money you can use brass jewelry, coins, or cut your own out of an aluminum baking sheet. If your school is fortunate enough to have a life-sized plastic skeleton do whatever you can to convince your principal that it won't get damaged and that dirt will wash off. Otherwise call your local hospital and ask if they will loan you one. Be creative and stick a spear point in the skeleton's side and sprinkle money around him. Get small and mediums.