Forget all that was here, before.
Thayer Street, really, is just another example of the importance of a stoop. It is located very close to two liberal colleges as well as, as BluePlanet says, School One, probably the most liberal alternative school I've ever known. Anyway. Where are all the kids gonna go when they don't have class to attend? Answer: Where there is food, drugs, and other people. And these people need somewhere to sit.
Store 24: One of the only Store 24s that closes is located on Thayer Street. And there is a ledge in front of the store. This ledge is actually very non-utilitarian, and it is only about 6 inches wide. It hurts your ass. Intriguingly, it is probably the most popular of places to sit on Thayer. One can usually find anywhere between 5 and 20 kids hanging outside.
The Gap: Yes, The Gap. They have nice ledge. Maybe 10 inches wide and a perfect height from the ground for sitting. The Gap is the chosen place for lots of aging bikers. On any given Sunday night you can find a very large group of them hanging out here and across the street, in front of Channels.
The Stairs Behind Esta's: These steps are on the corner opposite Store 24, on the same side of Thayer. They are well hidden by shrubbery and provide ample sitting room. They are a popular place to eat. One time, my friends and I sat there and no less than 6 people tried to sell us drugs in under an hour. We just look like that type, I guess.
So that's where to sit on Thayer. Now, Thayer street sucks if you aren't friendly with the locals. So make friends with them. They are generally nice. Find a group of kids who fit your chosen stereotpe. They're all there; emokids, goths, gutterpunks, hardcore kids, artists, hippies...