Or why not give her the money but have the school send you a copy of the account every month? That way you could know what was being paid and when. That would protect you against her getting a discount and not passing your share on to you.
Or, you could be super-prompt and drop a check off at the beginning of every month - this would only work if you were super-reliable though.
Also, ask yourself - is this the hill you want to die on?
