Designate an area in which your class will investigate food web relationships. Do not choose too large an area, as the web will become too complex. There is a huge amount of activity in a 20 meter square.
Cordon this area off, if possible (if it is within school grounds.) Otherwise, leave markers so that you can easily identify the area under investigation.
Do some preparatory research into the area before designing the assignment. Note down which species of tree and plants grow in the area, take soil samples to investigate insect colonies and look for tell-tale droppings of larger mammals. Set up a motion-activated camera to record any activity that occurs when you are not there, if your budget will stretch to accommodate such equipment.
Design worksheets for the class based on your own research. A good starting point is to have pictures of the main species occupying the marked area. Ask the class to put arrows between the animals and plant life, determining in which direction the energy is flowing. Include more species and sub-species, if the class is older.
Visit the marked-off area at various points during the year to see how seasonality affects the food web. Get a local wildlife expert in to speak to the class about the various relationships between the different types of wildlife. Make sure to always discuss what has been found in the area shortly after discovering it.