A bird bath, with shallow areas and dry places for the birds to stand, can lure in additional species. Bird bath heaters provide open water even on extremely cold days. Although space requirements are less in the winter, different species still have personal space requirements. Spread feeders out to maximize their use. A variety of foods are available, from sunflower to peanuts, corn to milo, thistle to safflower.
Many books detail the variety of foods, feeders, and bird preferences.
In general, though, you'll be better set with straight black oil sunflower. Much of commercial mixes are wasted by the birds. What better way to develop a working knowledge of the concept of habitat than by experimenting at a school bird feeding station. Your students' goals for determining the right feeder configuration may be based on the desire to maximize feeder use or to attract specific bird species. Each goal will have a different solution.