I may as well say at the start that I am not going to confront MIL about this. We don't see that much of her, she's incredibly stubborn and set in her ways, and it would only cause grief and guilt-tripping which I don't have the headspace to deal with. So I'm dumping it on you lot instead 
So we have two sons, 17 and 19. MIL has always been openly critical of our choices for them (state education, not "getting their names down for a good single-sex prep school" etc - plus we're too permissive, blah blah) and we've always politely but firmly shut it down and done our own thing. There have been a few flare-ups over the years, but DH has got much better at defusing these and asserting himself.
Our older son, who is autistic and has had a very rough ride through school indeed - serious MH difficulties in the past few years - is now at Oxford and doing really well. MIL has always favouritised him anyway, because he's very academic and that's what matters to her, but now she's positively fawning over him, visiting him every couple of weeks etc (she's only ever seen them once a year while they were growing up). Not a problem in itself, though DS2 does know that she is less interested in him and is a bit hurt by it and thinks less of her.
DS2 is a completely different kid - in every way. He's clever and quick, practical and utterly lovely. He struggled hugely with A Levels in the pandemic and was miserable. He has, on his own initiative, switched to a vocational course in a STEM subject which could lead either to uni or to an apprenticeship, depending on how he feels later on. He's doing really well at it, has a part-time job as well and is saving for a car etc. If I'm honest I was upset that A Levels didn't work out, I was nervous about him changing - but it was his decision, we talked it through as a family and did lots of research and he made his choice with our support.
So DS1 popped home to surprise DH for his birthday last week and let slip to me that MIL had been ranting at him about us. Apparently she's always thought it was negligent of us not to at least choose a grammar school. If we had sent DS2 to a better school he wouldn't have "dropped out" and would be more successful. It's really stung me. I'm not sure why - I knew these were her views - but maybe it's just the thought that she is bitching to our child about us being bad parents.
Am I overreacting? I need to shake this off so I can be polite to her next time I see her, but I'm bloody fed up.