To start with, the people that seem to have done no work could have done loads. You just can't tell from what people say. It's not even necessarily that they're lying, people just have a skewed perception of how hard they have or haven't worked and, at GCSE level particularly, people are afraid of letting other people know how much effort they've put in.
Secondly, people's real grades can vary hugely to their mock grades in either direction, there are so many factors that affect it. Also, as you're doing a new style set of exams your teachers don't necessarily know lots about how to mark them and in my experience people mark more harshly when they're not sure.
And finally, there's 4 more months til exams, that's a lot of time, if you work hard til then (make a plan, I love revision plans) and make sure you're revising in a way that works for you, you can go far.
Past papers are important. I know you're doing a new syllabus but the old syllabus papers are still extremely relevant (I'm doing a new syllabus for my physics and chemistry A levels and I'm doing ALL the old past papers). Just make sure you're aware what is on each of the old papers and what is on each of your new papers (ask your teachers, find the old and new specifications online) and find out what has been added on and make sure you write it down and revise it too as it will be missing from those past papers.
My final point is one that your teachers don't like to tell you because they're worried you'll get complacent.
GCSEs aren't THAT important. If you're going on to get further qualifications (I'm assuming with your grades you're thinking of A levels and university) the qualifications you get almost replace your GCSEs.
As long as you get what you need to get on to your chosen A level courses you're generally fine.
The only unis that seem to care about GCSEs are the very elite ones such as Oxbridge and even then they're probably the least important aspect of your application.