DD1 got 98% for her Maths SAT, went on to get A* at GCSE and is now predicted A at A-level, BUT she did go through a phase of struggling, and was not in the "top set" for GCSE maths at her GS. What is more, just as her sister who is now in Y7 is also discovering, the SATs really do not differentiate at the top end of the scale. After all, there is no longer a level 6 to challenge the really able mathematicians. This means that a score of 100% can be attained by a genius or by someone who is simply very good at maths, and everything in between. Both girls therefore spent the first part of Y7 at GS languishing around the 60-65% range. A bit of a shock to the system, but possibly one that was needed!
So yes, in our experience it is possible, but still requires lots of hard work and is not automatic!