Fact vs opinion may not be quite as clear cut as some including the teacher in question here think.
There are those who think the only facts are scientific ones. Others think there are other sorts of fact.
This may be a case in point. 'Girls should ... ' gives the game away. Read this as 'It ought to be the case that ... ' and you may spot that this is basically a moral claim.
OK, many people nowadays think morality is just a matter of opinion. But others think, no; certain things are just simply right or wrong whatever people think. Quite often this is posed in terms of objectivity or moral absolutism; roughly speaking, moral absolutism claims there are moral facts, whereas the opposition (often called 'relativism') claims that, no, moral claims are no more than statements of opinion, whether at a personal or societal level.
If you think there are no moral facts, you must be prepared to say that, for instance, it is not a fact that slavery is wrong. Or, perhaps more relevantly here, you might wish to say that it is not a fact that discrimination against girls and women is wrong.
The science teacher in question might well learn something about the complexity of the idea of a 'fact' from pupils such as the OP's daughter, it strikes me. And the young woman in question might well consider, on further reflection, that her initial reaction that it is a (moral) fact that girls and women should not be discriminated against regarding their choice of employment captured the truth of the matter after all. That is, it is true although not a scientific truth that girls ought not to suffer discrimination on grounds of sex.
[For OP's daughter: If you are interested in an example of who thinks there are moral facts (apart from some random on MN), perhaps you could look up some of the writings of JJ Thomson (that is Judith Jarvis the philosopher, not Joseph John the physicist). (She also wrote a hugely influential piece on abortion that you might come across in your search; no harm in that!)
Oh, and if you want to put your teacher on the spot (if he has anything to him as a teacher he should be pleased you do so), ask him whether his own assessment of what constitutes scientific method is itself a scientific fact or just an opinion.]