Schools aren't empty in summer holidays. Certain deep clean things take place eg carpet cleaning. Building works. Electrical refits. Decorating. Most days a few staff will come in. In secondary schools, obviously GCSE and A level results day, there are a lot of people in; but after those days, there will be staff and pupils making decisions about pupils' future courses/ resits etc.
Due to govt league tables which are based only on children born between that specific Sept 1 and Aug 31, exams have to be taken that academic year. I think it used to be, for example if they took Maths early in Y10, they actually reduced the percentage of top grades, because it would appear in the stats that they'd not taken GCSE Maths. Having a pupil in Y11 who has been long-term ill or is in the wrong year for bilingual language reasons automatically used to reduce your statistics because their attendance would count in the number of pupils in Y11, but their grades would not. Whether these rules have changed, I don't know, my knowledge is out-of-date.