The Smoking Tree (noun) is that one place at or near your high school where all the where stoners would go to smoke with minimized fear of getting caught. For example, at my high school the smoking tree is so big, it's leaves/pines conceal the smokers inside, so they will noy get caught.
The Smoking Tree refers to any place, not just huge trees, where people can smoke safely.
"Yo Tyrone, lets go drive to The Smoking Tree
during the lunch break so we can roll up this fat blunt."