@WonderfulSave
DC is currently isolating on the basis of a positive LFT in his class. I understand the child is getting a PCR.
Is this correct? I thought only the positive person and the family of that person had to isolate until the PCR was back.
Or is school being sensible, just in case? What's the official guidance?
This one is really a balance of risks.
On the one hand, in a time of high rising cases in the relevant age group, the ratio of 'true' to 'false' positives on LFT will be higher. False positives are, to a first approximation, a fixed percentage of tests taken. This percentage is large compared with true positives when case rates are very low, but much less significant when numbers of true positives and true cases are higher.
Thus at the moment, using LFT as 'a warning that we may have a case in school' and isolating from this point is sensible, as isolating rapidly brings the chance of secondary transmission (from those who the first case has infected on to a further group of pupils) down and thus hopefully limits the size and spread of the outbreak.
However, when case rates are very low, the risk that the LFT is falsely positive becomes higher and the damage to children's education of repeated 1 or 2 day isolation is not worth it.
I think the 'official' guidance is still 'isolate from positive PCR', but with the huge growth in cases in the 10-19 age groups, I think isolating from the positive LFT (false positives are anyway HUGELY fewer than false negatives) is a sensible way to try to keep outbreaks under control while many school staff are still only partially vaccinated.