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Class rank "doesn't help much": do you have an empirical basis for saying so? Let's say you are trying to see how well high school performance predicts college grades. I think the research shows that class rank and quality of high school program will enhance by a considerable margin whatever predictions you would obtain from GPA alone.
But that presumes that you are looking for a means to predict college grades. You haven't said why you want to sum up each student's high school performance. There is, probably fortunately, no single best way to "sum up" anything. That would turn something arbitrary into something universally right or wrong. Instead, systematic assessment hinges on the idea that we choose certain indicators because they are useful to predict or explain specific other things that are of primary interest. Perhaps you want to know how far a student will travel in school; how much s/he will participate in the civic process; or how happy s/he will be at your school. Each of these outcomes figures to have its own best set of indicators, i.e., its own set of factors that will furnish the best explanations or predictions.
Another way to say this is that the validity of an instrument or indicator is, strictly speaking, always described with respect to a specific use that will be made of it.