Metals belong to those elements that have easily shared valence electrons, and so exhibit high electrical conductivity. When metallic atoms are in proximity to one another they form metallic bonds, and, losing their electrons, form a lattice structure of positive ions held together by electrostatic interactions with the free floating electrons.You can further define metals by their conductance of heat, their ability to form cations (fundamental to their ability to conduct electricity), and their ability to form ionic bonds with non-metals. The bulk of the elements on the periodic table are in fact, metals, with some gradation through those materials that are semi-conductors. Metals are further split into rare-earth or alkali metals.
You can order many metals in pure forms from metallurgical supply companies but the trick is finding them in small enough volumes to justify purchase for a school science experiment. Many electronics supply stores will sell both aluminum or zinc blocks for use as an anode and pure copper wire. You can also buy aluminum in sheet form in most grocery stores. Building supply stores will also sell copper, zinc and aluminum flashing for use in roofing and window and door installations. Visiting a local scrap yard may also yield some interesting tidbits, but unless you are planning on refining your metals, be careful to purchase pure substances. Steel is in fact an alloy composed of several base metals and includes iron. You can obtain pure iron from old household implements, including cookware. Silver, gold, platinum and other precious are available in various levels of purity from jewelers. Small quantities of them can be useful for magnetic or electrical experiments.
"Eureka!" was the word Archimedes cried when he discovered a way of determining whether or not a crown was made of solid gold. Since pure metals have a different molecular density, their overall density can vary widely. An obvious example of this is the difference between the weight of same-sized blocks of aluminum, lead and steel. What Archimedes did was compare the displacement of water by set mass of solid gold with that of the crown in question. Since density is mass divided by volume, if one displaced more or less than the other, it means its volume was different, and correspondingly, so was its molecular (or elemental) composition. Similar experiments can be performed with a wide range of metals using a simple water bath set in a larger basin to capture the displaced water. Using the volume of displaced water, you should be able to calculate the density of your various pure substances.
Different types of metals have different electrical conductance based upon the configuration of their free electrons. Using combinations of nickel and copper and an ionic medium (a mild acid like lemon juice or vinegar) it is possible to generate electrical current. Experimenting with different metal types, such as nickel, copper, iron, aluminum and zinc and an acid medium can illuminate why certain metals act as anodes (or negative terminals or electron sources) and others act as cathodes (or electron sinks).
Similar to the way in which different metals conduct electricity differently. Testing the time it takes for heat to travel down a set length of metal can be another good experiment, all you need is a thermometer and a replicable heat source.