This is a genuine question - how is the density of the population not relevant? Iceland's population density is approx 3 people per square kilometre and the UK's is approx 274.
Overall population of Iceland is approx 375000 people compared to 66 million in the UK.
As I say, a genuine question.
From a personal perspective, working in a UK school, I can see that staff would be put at undue risk with a too early return as well as schools simply being unable to operate any degree of social distancing effectively. Having seen how difficult it is to socially distance 25 pupils (in school over the lockdown period) trying to do this for 800+ would be nigh on impossible.
There are also a lot of staff isolating for long term health conditions which would have an immediate impact on ratios.
Queuing up to drop children off - how long will this take in a school of 800 pupils? An impossibly long time! School would need to open at 7am to get everyone in via an orderly queue.
Staggered breaks - supervised by who? Schools will be already low on staff.
Staggered weeks - so parents will be in work one week, off work the next? Teachers will be double teaching the lessons.
I think the time to fully open schools is when they can open safely for all, staff included.