Re: using a common key value on both sides of a union ?

psql (9.6.7, server 11.3) on linux

I want to do something like this 

(intentionally bad sql but will illustrate the need)

select s.name,s.grade from students s where s.class='math'

union

select 'whole class', class_grade from all_classes where class=s.class

Of course it's that "where class=s.class" that's a foul.

In English, I want a list of each math student and their grade and then append one more record for the entire class, a record that comes from a different table but narrowed down to that one class.

I don't care if union isn't the right approach.  Anything that works is welcome !