Re: How to modify a page according to a selected tracker item
Perhaps you can try creating a field that corresponds to the student's class. Then either display it using the trackerfilter plugin to allow the teachers (I presume) to search by class through a dropdown menu or the trackerlist plugin with the filterid and filtervalue parameters set to a specific class. You can send in the sort parameter to your advantage. I realize this may not get you to necessarily BLOCK the teacher from accessing students from other classes, but at least they wont' be displayed (if you're using the trackerlist method).
I'm not entirely familiar with tikiwiki permissions really (because its very robust, yet complicated), and if it were me, I would do the above first and play around with the categories/tracker-local/group permissions until I get what I want.
I have to agree about the categories - its not entirely flexible enough for some purposes dealing with permissions, but its a somewhat recent addition to tikiwiki afterall and powerful for the most part.