If they dont accept the US student card, go to USIT in metro stop callao (cant miss it) they work with council travel, there you can buy a different card for about 6euros, which is less than 6 dollars, this card gets better discounts (movie theatres, train tix, and museums) and costs less. At the prado the cost for a student in euros is 1.50, which is less than $1.50. The reina sofia is the same i think.
good luck!