You've only been there 15 days.
How long was their previous teacher around? How long are their others teachers around for?
If staff turnover is high, which it tends to be in deprived areas with vulnerable students, they've likely had a constant stream of new, inexperienced, inadequate and unqualified teachers. If not just agency cover supervisors, which is effectively paying a random adult to come in and try to keep them in the room for roughly the same pay as a receptionist in a building firm with a far greater risk of being punched or called names. What's the point of being nice to yet another bloody NQT? You'll just start to like them and they'll fuck off and somebody else will be there.
For a large number of kids, school is the one source of stability in their lives. Rapid staff turnover (or gung ho heads wanting to fuck with what works just for the sake of change and moving people around) just takes away the last bit of security or reliability they had.
Screaming and shouting at them doesn't work. They get shouted at all the time. Speaking clearly and softly, however - they're not used to being spoken to with respect, whether amongst their friends, at home or, if they're wondering who the hell is going to show up for their lessons this time, certainly not at school.
The parents are often the main contributing factor as to why they are resentful, suspicious and prickly. You're weird for buying biscuits because they don't get treats. They don't get given anything without there being conditions attached. You know where you are if your mate has stolen the new teacher's biscuits - what creepy fuckery is there involved in accepting biscuits from a stranger who offers them to you?
You don't have to tolerate rudeness - but you do need to remember that what they say to you is absolutely nothing compared to what their parents, peers and some other staff say to them.
The most effective sentence I've ever used is a visibly irritated but calm 'have I ever spoken to you like that? Well? Exactly. So why would you do it to me?'. But, other than outright insults, the rest 'Oh, we want the old one back' (who they tortured weekly) and 'This is boring', etc, etc, is just trying to get a reaction from you to disrupt the class. It's not personal. You haven't been there long enough for it to be personal.