You should say "I sat in the back of the bus." It sounds strange if you say that you sat on the back of the bus. It suggests that you were sitting on top of the roof of the bus. Even if you did that, you would say "I sat on top of the bus", not "I sat on the back of the bus". Also note that "the back of the bus" represents the back half of the bus so you don't have to sit on the literally seats furthest back in the bus to sit "in the back of the bus".

You can say you were sitting 'at the back of the bus' or 'in the back of the bus'. 'On the back of the bus' means on the outside. For example: 'I was driving to work yesterday and got stuck at the traffic lights just behind a bus. On the back of the bus was a really funny advert for soap powder.'

i think u r right