Translation:School is scary at night.
If we could reinvent a Japanese language, it would be reasonable and perfect to design よるのがっこう as a night school. However, the actual Japanese uses 夜間学校 （やかん～; night + meanwhile + ...） for schools operating at evenings / nights.
When a new Japanese concept (especially in 'upper' sections of the society, e.g. laws, politics, or education) is needed, the word is usually formed by putting these kanji together instead of glueing some common words with Japanese particles. It is a bit like that in English one says 'astronomy' instead of 'learning of stars'.
～の～ is sometimes read as 'Somewhere in some time'. 夜の東京: Tokyo at night.
Another example: 櫻は春の嵯峨御室 紅葉は秋の高雄山 (The [best / famous] sakura / cherry blossom is [at] Saga-omuro in spring, and red leaves, Takao-yama in autumn. From Verse 50 of 鉄道唱歌. The both places referred are in Kyoto. Since the song was written in 1910, 櫻 is kept in a pre-reform character, which becomes 桜 now. ).