Why were the teachers like this back when I was in school?

Bear in mind I finished school years ago, but this is something that I still don't understand now.

Basically when I was in years 11 and below some of them used to treat me like crap on their shoes. However, once I'd become a sixth former, they were trying to be all pally with me. One teacher was attempting this after previously threatening me with a half an hour detention purely for putting my lipstick back on in the girl's toilets during lunch break (harsh I know) and another who was supposedly a special needs teacher but she wasn't nearly patient enough with people for that.

It seemed like they thought I'd forgotten the way they'd treated me previously. I felt like giving them a good mouthful at the time, but I didn't because I would otherwise have risked being kicked out, especially since excluding a 6th former is easier than kicking a lower down pupil out because it's non-compulsory education.

Why did this happen?