Berlin Secondary schools - help needed to navigate how it works!!

My daughters (twins) are just entering 6th grade and we need to start thinking about secondary schools.  I find it all quite stressful how it works here in Germany, the pressure on the children to get the correct grades for gymnasiums, and how even the integrated secondary schools in my area (I'm near Sophie School ISS) are usually oversubscribed.  The nearest Gymnasiums near mer are Leipniz and Robert Blum and I've heard you need practically perfect grades to get into those! 
So I'm not really sure what to do

The first thins is, I'd like to get information on the grade needed for each gymnasium so I can better assess whether my children would have a chance to get in.  I don't want them accepted into a school that's the other side of Berlin, which is what I've heard sometimes happens when the children don't get the grades for their preferred school. 
How can I find out what the average accepted grade for each school is (for the last few years would be useful)?  Is there a website that collates them all?  Or do I need to ask each school for that information individually?