Which comes first, a 5 year old's right to education or an 85 year old's right to medical care?
This wouldn't be the a question, though, would it? Hospitals are at risk of not being able to provide medical care to people of any age and for any condition. Cancer operations will be cancelled. With too many more covid cases, we risk people not being treated for car accident injuries, strokes, heart attacks, appendicitis...
We also can't manage this by deciding just not to treat people with covid over a certain age, even if it was remotely ethical to just cherry pick one medical condition not to treat, because the age would have to be at least 20 years younger than 85. It would not in any way look like just an extension of DNRs, or be implementable just by telling ambulances not to bring residents in from nursing homes. You would need barriers outside hospitals, and security, to stop people bringing in relatives from their homes. It would be messy, and ugly, and terrifying for anyone needing medical care of any kind.
And then, even if you succeeded in doing that, and enough people with severe covid stayed at home to keep hospitals more functional, but the virus was still spreading fast because we didn't want to close things like schools, then deaths would still keep increasing, just in ordinary people's homes. Again, this wouldn't 'just' be elderly people in nursing homes, tidily out of sight - it would include people like the grandparents, teachers and sometimes parents of the children whose education we're worrying about. Some of those deaths would happen in children's own homes.
I know you were asking questions, not proposing a particular solution yourself, but I think it's important we realise that anything involving deciding not to treat covid patients to avoid them filling hospitals would bring us to a modern day "bring out your dead" situation a lot sooner than we might think. Even if we could justify it ethically, it wouldn't be experienced by ordinary people as something tidily contained to very elderly people only.
What's most frightening, of course, is that if cases rise far enough and fast enough we might end up in that situation anyway, just by running out of capacity in hospitals.