Looking at this it does seem prevalence (or at least diagnosis) rates are higher in younger age bands, so the 1% statistic is misleading as it does not take into account the fact that autism was underdiagnosed for much of the last 100 years.
https://pmc.ncbi.nlm.nih.gov/articles/PMC10114511/
It seems to be more like 3% in children, so about one per class would be right according to that. I don't know whether the "3 in each class" or "10% of the school" are in schools with particularly high rates, or whether posters are counting children on the pathway (suspected) as well as officially diagnosed.
It is quite likely prevalence is higher than diagnosis figures suggest, because it seems to take so long to get a diagnosis.
I watched both of the Uta Frith interviews and read her article and she doesn't seem aware of the more recent research, which I find a bit puzzling. She states that a large proportion of people being diagnosed as autistic now do not fit the group that she was studying years ago, which is probably true. But I am not sure she is right that there is nothing in common between the groups.
I do find it believable that in the process of studying some phenomenon or group of people, it is possible to both miss part of the group, and/or to accidentally conflate more than one thing into the same group which later gets split apart. But there has been lots of research trying to split autistic people into different groups and none of them really seem to stick. If it was as simple as age of diagnosis that would have been established and I think it has also already been used as one of the criteria (the "four types" study seems like it is supposed to be more replicable than others?)
I don't think autism prevalence could be anywhere near 50%, that doesn't make sense - autistic people struggle with neurotypical social communication norms and often feel isolated among their peers, that would be unlikely to happen if it was 50% of the population. Even with something like sex, where men are seen as the "norm" and the default so norms are based on male bodies, it doesn't mean women feel isolated in relation to men, because we tend to talk to each other.