Your class will want to develop criteria that you will provide to the guidance office. Some suggested criteria items would be need, scholarship, leadership, community service, lack of other scholarships, etc. It is good to get input from as many classmates as you can.
The guidance office will then let the graduating seniors know that your class is offering a scholarship. You should provide an application form that highlights your desired criteria items.
Then the guidance office will select the individual that best meets your criteria.