Maybe do sub-ous under your “students” ou, like
/Students/Current (with the 50GB quota)
/Students/Alumni (with the lower alumni quota)
Then have people moved into /Students/Alumni with the criteria “if alum, but not current student or employee” and have people moved into /Students/Current with the criteria “if current student (regardless of alumni status)”.
I’m really not familiar with Active Directory, but I know that many group management tools (like InCommon Grpuper, for example) can easily do that sort of Boolean logic on groups.
Hope that helps,
Ian