Problem with volatile function

So my understanding of volatile functions is that volatile functions can
return different results given the same input.

I have a function random(int, int) that returns a random value between
$1 and $2. I want to use it in a query to generate values. But it only
evaluates once per query and not once per row like I need it to.

-- This always returns the same value
SELECT ts.sis_id, bldg_id, f_name.name, l_name.name
FROM tmp_students ts
JOIN names AS f_name ON
ts.gender = f_name.gender
WHERE f_name.counter = random(1,300)

--As does this
SELECT ts.sis_id, bldg_id, f_name.name, l_name.name
FROM tmp_students ts
JOIN names AS f_name ON
ts.gender = f_name.gender
AND ts.counter = random(1,100)

-- This generates different numbers
SELECT random(1,100), s.*
FROM usr_students s