That happened to me to, but I'm home schooled now.
I caught a bus last year to school.
Normally there are alot of people standing, but sometimes there are seats left.
If there are seats left, I normally ask if I can sit in the available seat, but I always got a no.
So, to overcome my SA sometimes I would just sit somewhere without asking if I knew it was available. I got told to go away. Even if it was the last seat I got told to move, so sometimes I was the only one standing on the bus.
On the way home, the bus came to my school first (in the morning we get dropped off last) which there are only about 5 people from my school on that bus.
When the bus picks up the students from the other schools, even if the seat next to me was the only one free, no-one would sit there.
Sometimes the junior boys would play dares on one another and say 'i dare you to sit next to that girl', meaning me.
It was a big deal. I used to put my bag on the other seat, but like you did, I started putting my bag on my lap.
Some people sit on one another if there's no seats. But there's always one available seat next to me.
I don't know why no-one wanted to sit next to me.
I have very good hygiene, I get hit on alot so I must not be an incredibly ugly person either (even though it makes me very uncomfortable).
I also tried smiling to people as they get on the bus and pass me.
I would also like to know why these people avoided me at all costs.
You're not alone with this