First and foremost is to understand how students jointly plan the final mark. It may be a couple who communicate with each other person and the whole class (often happens that way), and even the entire parallel when teenagers in her friends. From this information and assumes the scale of the planned event.
Then, identify the number of people, you can think about the venue. If your budget is limited, it is good to hold the exhaust in any place of public resort: Park, wooded area, waterfront or anywhere else. Money is not very much, basically it will have money for food/drink. If resources permit, the range may expand with clubs, cafes, entertainment events, and the fact that enough of the imagination of parents and their children. In this case, the money will take longer, but parents will not have to headache about what will the children do, what they will eat and drink, what they will do.
You should think about how to get to the venue. Most often chosen public transport. Cheap and cheerful. Angry especially, because often the children have nowhere to sit, and they themselves often become a headache for the conductor and surrounding the children adults. Therefore, a rational way for a day or two to remove a bus/minibus which will be able after the official part of the school to meet the children, take them to the place, and then breed them at home.