Sorry for nerding out, but …
There are two official romanization schemes, which are Hepburn (the most frequently used romanization system, especially outside of Japan) and Kunrei-shiki (which is almost exclusively used inside Japan, and more typically for specialist purposes if I’m not mistaken). In neither is a long o-sound written with the sequence ou, as you can often see in such spellings as doujinshi or Touhou; these are examples of what is called wapuro rōmaji, or “word-processor romanization”, because this is how you would type the kana sequence おう (found in such words as 学校(がっこう)“school”) using an input method editor (IME) with a QWERTY keyboard.
Instead, both official romanization schemes use a diacritic to indicate a long o, either a macron (ō) in Hepburn, or circumflex (ô) in Kunrei-shiki. Therefore, the word 学校 should more properly be romanized as either gakkō (Hepburn) or gakkô (Kunrei-shiki).