Moulds vary very little between the type of material that is to be cast into them. The principles for casting chocolate are almost identical to that of casting steel. All moulds need to be machined with relief angles. Unless cores are to be used moulds cannot be made with reverse angles due to the fact that as the material sets a rigid mould would then lock in the casting. There are alternatives in mould material like the use of silicon moulds which can be stretched and deformed to allow a complex shape to be removed. Alternatively lost wax or biscuit moulding can be used where the actual mould is destroyed in large rolling mills can be used but each of these have higher costs and are not easily available to schools. All the following moulds are designed to be made on a CNC milling / routing machine in whatever material is available (The best being Model Board). The master having been machined is then vacuum formed to make the working mould / moulds. The finished vacuum forming can then have either chocolate / jelly or soap cast into them using melt and pour. A microwave is very good at melting chocolate and soup. Just a tip, cooking chocolate is a cheap way of making castings but if you want to make a product that would sell well at a school fete invest in a good belgian chocolate designed for chocolate fountains. Fountain chocolate has all the oils to make mould separation easy and its taste and texture is untouchable when you don't have the facilities to temper the chocolate. (Tempering alters the crystal size of the chocolate and stops the texture feeling cheap and grainy, Yes there is a lot of technology in chocolate making.) If you want to go down the route of high quality chocolate it is worth visiting a local home brew shop to purchase some liqueur flavourings, they have all the flavour with no alcohol so they are ideal for school use. Flavours like Cherry Brandy and Amareto mix well with the molten chocolate and add a whole extra dimension to the experience. Finally on a safety angle the safety of casting chocolate is far better than steel at 1100 Degrees.