I had that problem last year, and you know what I did? I quit. It was worth it because I just started my first semester at an ivy league school.
What's more important? Taking a test that'll determine which college you get accepted to, which is your entire future? Or going to you little job, which you won't work there forever. And besides, they will replace you the second you leave.
That job is nothing. You can get another one easily, but you won't get another chance to take a [paid] test.
Please take the SAT. If your boss doesn't understand, threat to quit or call in sick.
Don't destroy your whole future for a dumb job that you'll quit eventually.