It is difficult. You haven't signed a contract but as you say, they are saying that the offer of the job stands.
Employees are entitled to be off work and to be paid as their contract states, when their health prevents them from working and they have medical evidence for it. Usually your entitlements are lower in the first 2 years of employment, but often in teaching you have continuous service if moving from one employer to another directly without a gap. I would check this.
There are 2 issues. One is how you feel about it and one is the legal position.
If you feel you can't accept a job you haven't started knowing you can't work from the start, then offer them the chance again to rescind their offer. You don't have to do this but if you feel better doing this you can. It shouldn't be determined by what people on MN or what you think other staff memebers will think but your own personal feelings.
Know that legally you are probably entitled to officially start the job on sick leave. Councils are often very hot on the letter of the law and you can involve your Union if in any doubt about their handling...or it would be a good idea anyway. This happens at non-Covid times and employers find it a bit tricky but is just something businesses have to suck up. People accept a job and then find they are pregnant and by the time they are due to start are about to go on maternity leave, or an illness comes on or whatever...Places of employment have to meet the law and be financially prepared for the costs and difficulties which come with this.
Of course, good, clear and early communication makes a big difference. If this information was available or likely a few weeks a go and you didn't spell it out to them, being told the week before school returns will no doubt piss people off....not about the issue itself of you not returning, but the late notice. But sometimes it's not possible to know these things in advance and they just do happen at the most inconvenient time and again employers have to deal with it.
You don't need to stand down from the job if you are entitled to it and to be paid. Step aside from being worried about what people think....it really isn't the issue. All you can do is to communicate honestly and clearly and in good time (and you know if you've done this) and be co-operative and also to look out for your own rights and interests and make sure there are people alongside you who will do the same...because unfortunately, if you don't do it yourself and have others alongside you, sometimes people will override those rights.
Try to go the your meeting with suggestions of things you can do from home to be useful to benefit the other staff and reduce their workload in other areas. Having some sense of the timescale involved will also help. Be proactive and not passive.