Do medical schools put some type of preservative in them?
Not an expert, but I’ve heard the very simple method is to wash bones with mild soap and water, then let soak for days, if possible. Some will use Hydrogen Peroxide, but not that 3% solution you find at the dollar store. There are several stronger concentrations. Place in a flat tub, fill with Hydro/Perox, fill and cover for a day or more.
Don’t use bleach, as that will break the bone structure down.