It seems ideal to be planting seeds in the fall garden with students, but it can be too rushed at the start of year when it's important to take the class time to establish expectations in the school garden and create routines for caring for plants, insects, and tools.
However, even a few weeks into school, there are crops that can be planted in late September in our 7a zone. There's still time to plant radishes*, baby turnips*, cilantro, spinach, lettuce, and cut come again greens (mizuna, baby kale, etc). Cover with plastic hoops before first frost, then wait until early spring for the greens to start growing again.
*(a cool experiment is to harvest radishes or turnips, slice off the bottom half of the root for eating, then replant the root and top for winter growing and possibly seed saving.)
Even with adequate warmth, plants depend on length of sunlight in the day to grow. Eliot Coleman, master of winter growing in the Northeast, coined the term Persephone Months for the period of time when days are less than 10 hours in length and plant growth slows significantly. This period of time varies along latitude lines; our Persephone Months start on November 14th. By that date we want plants to be at least 75% mature in order to survive the winter. Read more about Persephone Months here.
Johnny's seeds has a handy chart for winter harvest planting times based on the last 10-hour day of your season.
Cover crop option
Try growing Austrian Winter peas! These are just like field peas used for pea shoots but hardier. Just like field peas they add nitrogen to the soil and prevent erosion from winter weather. You can sow them 2 to 4 weeks before first frost.
It's not too late!
If all else fails, plant garlic. This bulb goes dormant in the winter and sends up fast-growing green leaves in the spring, perfect for weekly observing and measuring when the earth still seems barren. We have had success planting garlic as early as October and as late as December.
What else have you had success with in the winter garden?