There is statistical evidence of outcomes at A Level in different subjects,mfrommdifferent GCSE starting points.
There is evidence that in Maths, unless you have a 7 or an A or above you are unlikely to get more than an E and hardly anyone exceeds a D. Very many fail.
In some subjects it will be possible to achieve a decent grade with a 6.
Most schools which have 6th Forms rely on both GCSEs and teacher knowledge. Funding issues aside (which push schools and colleges to take students into courses where the prospects for that student are very poor) teachers often know about a students capability and work ethic and if a GCSE grade achieved is a lucky success or a disappointment and can advise based on that. Often those with a low 7 at maths GCSE aren't actually suited to further Maths study.
Parents should ask schools/colleges what the likely A Level outcome is in different subjects from the starting point.
I would say 4s and 5s should definitely not be doing A Levels and those with a string of 6s might need a combination of 1 or 2 A Levels and something else with it.
The problem is that kids and parents want them to do A Levels and schools need students in place to get funding and so people are let onto courses where good grades are unlikely. Depends what you want ....if 2Es is a good outcome, then it might be achievable, but starting out with a series of 5s and 6s at GCSE under the idea that you are likely to get all Bs at A Level and have a wide range of competitive courses available to you is just daft.....but students don't know unless they are told and people aren't always great at telling them unless pressed pretty hard.