What I'm guessing is that you want to select an Opleiding to be on the form so that the user can choose a specific Opleiding he wants, but he only should see the options that the selected school has. I think you are missing an association.
In my screenshot your user can choose 1 opleiding on his form.
I'm now doubting whether this will work, because the XPath will probably not look in context. To be sure you should set an OnChange Microflow on the selection of School
And use a microflow to select the "Selectable Objects"