Revise for your mocks like they are the real thing, but don't stress about them!!
Teachers and sixth forms know that you will probably get a better grade in your actual GCSEs
Generally sixth forms just want to see that you are on track to pass english and maths, and that you are doing decent in the subjects you want to do
Your teachers will use your mock grades for your predicted grades BUT don't stress. If you get like a 3 or something in one of the subjects you want to do, and you need say a 6 to get into the sixth form, don't worry. Your teachers will most likely also base the grade off of your work in class, class tests etc. They aren't going to hold it against you if you just had an 'off' day, etc. GCSE predicted grades are not very important. They just want to know that you are on track to getting decent grades in the subjects that are relevant to what you want to do
ITS IS IMPORTANT TO REVISE THOUGH
It helps you practise revision for the actual GCSEs, and allows you to see what areas you are stronger / weaker on.