A small site of 10 square meters or so will naturally be easier to prepare than a large field or parking lot sized area. A small site can be worked and planted using common gardening tools, whereas a large area may require some power equipment like tillers or even tractors. A large workforce, though, can reduce the need for such machinery. This is another factor which again points to getting as many involved as possible. In any case, even large parcels can be handled with just a little more planning and organization up front.
The easiest way to prepare your parcel is to somehow turn the soil over. This destroys the plants that are currently growing and generally starts you from scratch. Working the soil takes time and effort, but it is much easier to monitor changes and identify what is growing when you start from a clean area.
To turn the soil, over use a garden tiller or even arrange for a tractor to do the job for you. Most school district have someone in charge of buildings and grounds, and don't overlook the possibility of their help. If using power equipment is not feasible, using volunteers or students with shovels, forks and rakes can do just as good a job.
Rake and level the area to remove large clods and stones. (Your students will learn the derivation of terms like clodhopper). Remember, this is the ideal time to add soil amendments to improve soil structure, so you want to have them ready for this stage of the restoration. If the soil in your parcel lacks organic material and looks like it is made up of just clay, adding peat moss would be a good idea. Add a thin layer of peat (1/4-to1/2-inch deep) to the top of the ground, then mix it in. The addition of chemical fertilizers is not necessary; in fact the fertilizers actually spur the growth of weed species more than the native plants.
If working large areas is a problem, because of a lack of equipment or your workforce is too small, you can try an alternate approach where you use herbicides to chemically kill all plant growth in an area. It is important to remember that these are obviously toxic chemicals to the environment and should be used carefully, with directions followed closely. Another thing to watch for with herbicides is their latency period. Make sure you discover how long they will continue to stop plant growth so you can plant accordingly. If you are going to use herbicides, it might be better to apply them in the fall in preparation for next spring's planting.