这是一个由主语"it"、谓语"is"和宾语从句"how important it is to work hard at school"构成的句子。其中，宾语从句"how important it is to work hard at school"作为整个句子的宾语，由连接词"how"引导，包含主语从句"it"、谓语"is"和不定式短语"to work hard at school"。