While waiting for their answer you can check the following website:
www.machupicchu-tour.com is an online travel agency, some travellers have bought their tickets thru this company when the MP website crashed.
They will charge you a fee and they will request the following:
To have a student discount, you must send now a copy of your International Student Identity Card (ISIC). Our Agency cannot make the reservation at the authorities' office at a student rate without this copy (forbidden).
By agreeing to continue, you certify that now from this page you will send a legible copy of the original ISIC card, valid on the date of the trek. Sending any other document which is not an ISIC card (another card, certificate, written proof, forgery or any other) is an infringement to our terms of purchase which will then be applied.
Hope this helps you for the moment