Good night! 70,000 big ones for a 15x30? I can't hardly wrap my brain around that. anyway...
Is this being used by a garden club of students or an actual class.
Plants plants plants. But 15x30 isn't really a huge amount of room to work with, either, so, my thoughts are along the same lines plant sales of vegetables and flats of flowers if at all possible. Mixed combination containers for Spring sales if those sell in your area
( add those soil moist crystals to the soil to retain moisture). You don't have a lot of room, but what about hanging baskets? all the plants started from seeds are very doable.
Are you going to try to utilize it yearish round or just from Jan to April? Or whenever your Spring market is? You could try some container growing of vegetables or a small hydroponic system just to get the idea of how hydroponics works. Is the floor finished (concrete) or sand/gravel? that may change how you use it. Check out MHP ? gardener out of Virginia to see what his project are going on in his houses. simple ECONOMICAL methods and ideas.