Constraints to Guarantee unique across tables with foreign key?

I have two tables like following:

create table attendancereport (
id serial unique not null,
staff_id integer not null references staff(id),
schoolyear varchar not null references schoolyear(year),
students_id integer not null references students(id)
);

// schoolyear.year in format "2003 - 2004".

Create table attendancerecords (
attendancereport_id integer not null references attendancereport(id),
schoolday integer not null references schooldays(day),
attended bool not null
);

// schoolday.day in formation YYYYMMDD as in 200301222 for dec 22, 2003.

What I'm looking for is a way to create a unique( ) across tables via the
foriegn key, something like

Alter table attendancerecords
ADD unique (schoolday, attendancereport.students_id);

so that for a given student, there can be one and only one of any particular
schoolday. Can this be done with constraints?

-Ben

--
"I kept looking around for somebody to solve the problem.
Then I realized I am somebody"
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