Etching is a process that is not carried out in schools as much as was once the case, when etching of printed circuit boards (PCBs) was a common process. It still, however, has the potential for some interesting chemistry or IDL work.
You can find more information on etching here.
While there are various different methods for etching different metals, the one most suitable for schools is the etching of zinc. This can be etched with a copper sulphate based etchant. The process produces no nasty fumes or hazardous by-products and can be carried out quite quickly at room temperature.
Details can be found here.