Espee - they love school - that is the most important thing. What tends to happen is that they take a few GCSEs when they get to the top of the school, then go on to sixth form for A-levels. And thence to university.
Academic progress is different, but when you say not up to standard we have to ask whose standard? The academic pressure in mainstream school does have certain standards, but I question whether they are always the right ones - and I am a school governor, so see a lot of it.
Children from Steiner schools are not at a disadvantage when it comes to academia - if that is the route they choose, they will have taken enough GCSEs to get into sixth form and the universities are more interested in their A-levels.
I have a GS who hated his mainstream school, and did no homework ever, but he left with a few bits of paper - enough to get into college (to do something he loves) and he is now going on to university on the strength of his college successes. He used to say to me that he preferred to do the detention rather than do the homework! I never worried about him - he is very intelligent, had identified where his priorities lay and proceeded to do what just was necessary to get where he wanted to be.
So....what I am saying is that there is more than one way for children to get to where they need to be, and it does not have to be the tunnel that is represented by mainstream education. From my cintacts with local sixth form college the message I get is that he Pupils from our local Steiner school are very welcome - they tend to be more mature, get on better with others (both adults and peers) and have the skills to manage their own work in a sensible way.
So, please do not worry about your GC - take joy in their happiness!