One way is to have scenes of students walking through piles of fallen leaves on the school grounds. Or maybe a character starts a new hobby, like apple picking, to embrace the autumn spirit. You could also mention the smell of pumpkin spice in the air around the school cafeteria.
Well, you might incorporate school events like a fall festival or a football game under the autumn sky. Also, describe the characters' moods as they enjoy the season's beauty. Maybe someone has a special memory related to autumn.
You could describe the changing colors of the leaves, the cooler weather, and students wearing cozy sweaters.