As you and your school write your own seed stories, you will save more and more of your own seeds to plant. However, you may be uncertain about obtaining seeds for starting without buying many, many packets. Here are some recommended sources:
Local seed stewards and gardeners who are already maintaining and preserving locally-adapted seeds. Collaborate with and encourage these people if they are already in your community.
Your local seed library may have a curated selection of purchased and/or locally-grown seeds. They are a great collaborator and provide a venue to share seeds from your own school garden with the wider community. The Seed Library Network maintains a map of seed libraries worldwide.
Other schools writing seed stories. Trading seeds and letters allows students to interact with others in the region or across the country.
Local seed growers and suppliers. These local businesses may handle locally-adapted seeds.
Going to Seed is a grassroots effort of home and community gardeners creating landraces suited to their own micro-climates sharing seeds. Seeds from a variety of crops are made available for a nominal cost each year.