Why has there been many cases traced to pubs and restaurants where people were well over 1 m, even 2 m away and contracted the virus as it circulated in the air yet in schools they're saying in a class of 30 and a teacher for an hour, if someone tests positive only maybe 5 students go home now? So the other 25 and the teacher stay in school and spread it to many others?
They are changing the guidance because they haven't got enough tests and don't want loads at home isolating and not going to work. Simple as that.
Sorry but you're running away with yourself and that last suggestion is just nonsensical.
Guidance on schools has been published for some months (updated regularly, most recently 10 Septemeber). The risk assessment element has long been in there. It's not new, it's basic principles of health protection.
Health Protection Practitioners understand transmission of infectious diseases far better than you or I do. They do the job they do for fuck all money, despite its importance to society, to protect people's health. They have no incentive to do anything to aide the Government, or the governments contractors Dido/Serco/Deloite, who've cut their budget and headcount every year for the last seven years. It's not some wild conspiracy.
Think about it, they wouldn't send anyone home at all if your conspiracy was the case would they?