I don't necessarily think it's a bad thing for this teacher to make pupils aware of challenges they may have to face in the future...what is peculiar is the way she presented it as a fait accompli, something that has no chance of ever changing. She seems to have based her thoughts on the restrictions she faced as a professional woman, in a career that started 30/40 years ago, rather than acknowledging that, but also noting how much things have changed since then, and what her pupils could do to progress things further for themselves in the future...so suggesting that there is still a fight but it is one that can be won.
As others have said, the male/woman working issue is a societal one before a biological one...what she is ignoring is that, as other posters have said above, the people currently in charge of perpetuating these stereotypes (CEOs, MPs, TV producers and writers, Daily Mail journalists) will be retiring soon, and these girls will have the opportunity, as they progress in their careers to choose what becomes the new normal.
Therefore if I were the headteacher I think the best thing she could have said is to encourage the girls to break those conventions, e.g. 'Do what's best for you, and don't judge other women for their choices.' Don't judge your mothers. Don't judge your boss for what she had to do to attain her career. Don't judge your schoolfriends when you get together for a reunion 20 years from now for what they have, or haven't done.
Or, simultaneously. 'DO judge any man who sees child rearing as a solely male role.' Don't marry men who automatically assume that you will give up your career. Don't marry anyone without discussing this first!!! Don't praise your male colleagues for leaving work early to attend a Christmas play or pick up a child, and bitch about female colleagues for doing the same thing. Ask your male colleagues, brothers, friends as many questions about their children as you do to your female acquaintances.
Societal pressure doesn't just come from 'external' sources, TV adverts, papers, laws passed, etc. It comes from the people we know in real life.