Any sort of seasonal flower, whether it's spring tulips, Easter lilies, fall mums, or poinsettias, can act as the foundation of a great fundraiser for your group.
Get started by finding a local nursery prepared to sell flowers wholesale to your group, so you can take advantage of the discounted prices. You'll find that this sort of fundraiser works best as a pre-order affair, but it's a good idea to have at least some extras available so you can make any last-minute sales that you need to. You might also want to run the fundraiser multiple times each year to take advantage of the changing seasons.
Arrange with the nursery you're partnering with to decide if you are going to have the flowers delivered or whether you need to go and pick them up. If you do choose the latter, make sure you use a heated vehicle so that the cold air doesn't shock the plants.
You can advertise your flower sale using flyers sent home with students and through advertisements in local newspapers. Designate a pickup day and specific time for everyone to come and get the flowers they've ordered. You might also want to send each plant to its new home complete with a thank you note and care sheet.
For more details on fundraising with flowers you can see this article.