no medical expert is saying opening a school with 180 pupils running around as you put it is OK. The government are advised and then make a judgement, if you believe Boris' line about following the science then fair enough, but it's much more complicated than that.
@Redcy might have gone a bit over the top when he said they were lying, but he's spot on about the thought process. Schools have to reopen to get people back to work. The judgement is which will do more harm to society a continued lockdown and economic devastation or easing lockdown in the hope of helping people financially but putting them at risk from a public health perspective.
If you really want to go with the medical experts, the WHO have published 6 goals for countries to achieve before they ease lockdowns (bold bits are me), they are...
1. Disease transmission is under control -
Not achieved in the UK
2. Health systems are able to "detect, test, isolate and treat every case and trace every contact" -
Not achieved in the UK
3. Hot spot risks are minimized in vulnerable places, such as nursing homes -
Not achieved in the UK
4. Schools, workplaces and other essential places have established preventive measures -
presuming your 180 kids will be socially distanced
5. The risk of importing new cases "can be managed" -
This is why they've introduced the 14 day quarantine rule for people arriving in UK
6. Communities are fully educated, engaged and empowered to live under a new normal -
debatable if this has been achieved, but we'll stay alert
So if you really want to go with the medical experts, you should be agreeing with everyone who says it is too early.