"Kayou" clearly shows that you attend the school. "Iku" can be used also when you just go to there on some errand, depending on the context (such situation would be rare, though). Anyway, "Gakkou wa doko desu ka", "Dono / Doko no gakkou ni kayotte iru n desu ka", "Dono / Doko no gakkou ni itte iru n desu ka" are all correct.