It's kind of easy actually. I was way thinking too much and wanting too much. The elements I've chosen for my research are that big that only a group of researchers which spend years on it can accomplish it. Well, I'm not trying to do that, so I've cut in my research variables. It really clears the air, for I can see the path again :)
First, I've had to let go of some very interesting variables namely giftedness and under achieving. These elements will pass by, but only as a really small piece and not as a main variable.
And, finally my (I'm afraid to call it definitive, but it is actually definitive) main research question:
''In what way does the placement policy of secondary schools, in the field of determination and student counseling, affect the throughput of students?''
(In Dutch: ''Op welke wijze heeft het schoolbeleid op het gebied van determinatie en leerlingbegeleiding invloed op de doorstroom van leerlingen?'')
In which throughput stands for the numbers of: grade retention, demotion (and up-promotion to another higher school level) and drop out.
So, the dependable variables are: grade retention, demotion and drop out.
The independable variables are: placement policy and student counseling.
I expect to finish the final research proposal in a couple of days!