this years GCSEs will be significantly harder than last years
Where on earth have they got this from? Edexcel at least have said that this year's maths Foundation paper will be easier than last year's. They can't possibly make them harder given the low grade boundaries, that would be madness.
Do the teachers possibly mean that they are expecting higher grade boundaries this year? That's possibly true because students will be better prepared, but if the papers are harder, the grade boundaries will be lower.
Grade boundaries are decided by comparable outcomes. The grade transfer from letters to numbers was decided by pegging 4 to C, 7 to A and so on. The year-to-year grade boundaries are decided by looking at KS2 results and the previous year's results. If students with a particular KS2 profile last year got a 4, then this year, students with the same KS2 profile should also get a 4 on average.