Public school teacher, I'm guessing? Hopefully math and sciences. We (ganga users) don't have a large enough representative sampling in those specific fields.
So, since your brother's planning on using your piss to CHEAT the test, I'm guessing he'll be really lenient with the students, when they do the same thing?
Lots of fluids (remember, too much water is toxic to the body! And, not just when you fall into it!) Electrolytes are actually important to the body (and to passing a piss test.) Not smoking really helps. As does eating a nice steak breakfast with a pot of coffee and a couple litres of cranberry juice, washing down a couple B-complex vitamin, and a creatine/B-boost in your last litre before the test.
A dilute sample is sometimes considered a fail. If it's yellow, stinks, and has creatine in and a high level of protein in it, it's not "dilute."