Contact your Union rep who will issue the school with clear guidance on this matter: you are not (unless you're in an Academy or private school) required to set work when you are off sick. If you are off on a course or a trip, you must leave work. If you are off sick you are too sick to work (geddit, school?) and that includes setting work for others to teach. Your school is not even entitled to contact you at home unless you specifically give them leave to do so. Your line manager is paid to set work for absent staff. If you don't fancy a confrontation with your line managers over this, ask your Union rep to have a word and ensure that no comment about about this goes on your record. I daresay an apology will too much to ask but you should not have been told off for not setting work during a period of sick leave.