If you wish to observe natural behavior then you will want to provide the spider with space and habitat similar to the one from which it was collected. You don't need to purchase any thing, a plus for classroom use. Old mayonnaise jars, large clear containers minus contents from Price Club, yogurt cups with clear lids, old pictures frames for web spinners, etc. are just a few ideas. For a touch of color buy a few silk flowers at a yard sale or McFrugals. Food can be provided relatively easily, a must for busy teachers.
Fruit flies are easy to get and maintain.
Save them from your genetics experiments. Ask the fly lab at your local universityi for their leftover flies. Keep them going with moist food. You can also make a less smelly food with Jell-O, corn meal, oat meal, and sugar or honey. For larger spiders, try raising house flies.