That Sux! I mean, thats the last thing you'd want...for your arm to hurt. im in year eleven but some suggestions:
Keep your arm in water for at least 15 mins...warm or cold.
Rub VOLTAREN EMULGEL and soothe out the pain
Sleep!
Get a stack of tissues, get some moisturising cream, apply a thin layer onto your arm, and then with the tissues, wet them, and apply them as a light Cast.
As A Metrosexual Male, Its My Duty to inform you that before your test, look at something lilac, rub salt water on your rists for relaxation, and EXFOLIATE!