You'll improve your chances of successfully germinating tree seeds if you gather them as they mature, just before or after they drop from the tree. Have students inspect their finds, then discard those that have pinholes that could be caused by worms or that have other obvious damage. Another trick is to empty seeds into a bucket of water and discard the floaters, which are less likely to grow.
Before trying to germinate tree seeds (and other wild seeds), you need to brush up on dormancy. In an inactive state, seeds can survive adverse conditions such as freezing temperatures, drought, or fungus attacks, which a seedling could not. To prevent germination until conditions are favorable, plants have developed some sophisticated control mechanisms. Two common ones are described below. Consider challenging your classroom scientists to take a stab at breaking tree seed dormancy.
Hard Seed Coats
These prevent water being taken up until conditions are favorable. You can encourage germination of seeds with hard coats -- honey locust, honeysuckle, magnolia, and redbud, for example -- by scarring or chipping a small part of the coat away, or rubbing the seeds over a nail file or sandpaper.
Many tree and shrub seeds are naturally dispersed in late summer or autumn. In a temperate climate, if they were to germinate at that point, the plants would be unlikely to survive unfavorable winter conditions. Certain species inhibit embryo development until the seed has been subjected to one to several months of cold temperatures. You can experiment with breaking this type of dormancy by placing seeds from temperate climate trees (oaks, crab apples, maples, and so on) in moist peat moss or paper towels, then putting the mixture in a plastic bag. (You may also have to scar those with hard seed coats.) Leave the bag in a warm spot for a few days. This will allow seeds to take up water and swell. Then place the bag in a refrigerator for eight weeks before planting.