"Gakkou e itta hou ga ii desu."　＝　you should go to school ~ suruhougaii means speaker's advice to his listener . so , this means the subject of the sentence with advice is "you"(anata) not I(watashi) I should go to school = "watasi ha gakkou he ika-nakerebananai " or "watasi ha gakkou he ikanakerebanarimasen"

I would try it as follows: "Gakkou e itta hou ga ii desu."

I should go to school = watasi-wa gakkou-e ikanakerebanaranai