该校 is a formal written expression meaning 'this school' or 'the said school'. It is used in official documents, news reports, and academic contexts to refer to a school that has already been mentioned or is understood from context. The 该 (gāi) is a formal demonstrative meaning 'the aforementioned' or 'this', and it attaches to nouns like 校 (school), 公司 (company), or 地区 (region). You will not hear this in everyday conversation — it belongs to formal written Chinese.