HELP please, marking records (with example)

I have some student evaluations that I need to display in a portal but I only want to display the last evaluation for each students based on a search date (a global).

I have a script markings these records right now, but would want to eliminate it (no buttons). What's the best way to achieve this ? I'm trying with a custom function but I'm stuck on how to do it (not enough experience). Must refresh quick if report date changes and be indexable for use in the portal relationship.

Thanks

Claude

Ex. All last evaluations before 22/02/2006 (the global) would give

ID_________eval date___[color:red]Calc result

student1___28/02/2006___0

student1___21/02/2006___[color:red]1

student1___11/02/2006___0

student2___28/02/2006___0

student2___21/02/2006___[color:red]1

student2___11/02/2006___0