What you need: A net is a good idea, but you can conduct a very successful safari without one - you can just look at creatures or nudge some into a plastic tube. Another collection method is to take an upturned umbrella and something to "beat" the trees with. The insects fall into the umbrella and pupils can look at them up close.
Plastic tubes are safer than glass ones. Richard Jones particularly recommends those used by haberdasher shops to sell buttons (they have holes in the lid where the button is "sewn" on). Many will give them to schools or sell them very cheaply, or you can buy plastic boxes from educational suppliers, but they are more expensive.
Magnifying glasses or hand lenses are especially useful with older children. Richard Jones prefers the old-fashioned Sherlock Holmes type, but there are many kinds available. Petrie dishes are needed for showing the whole class a really interesting creature. Glass ones are clearer, but plastic ones don't get broken.
You also need an identification guide to bugs. There are lots of these available from educational suppliers. If you don't know what a creature is, draw a picture or make notes and look it up when you get back.
You can bring insects back, but keep the following in mind: Insects are very delicate and few will survive overnight. Most flying insects like bees and butterflies will exhaust themselves flying around, so you should leave them in their natural habitat.
Creatures you can bring back include spiders, ants and beetles, but it is best to release them on thesame day.