Current Sci teacher here, been teaching for more than 15 years.
At GCSE, specifications will state XX/XY for biological sex, AQA also has a couple of examples of DSDs. You will not be pulled up for staying male/female XY/XX, as it is in the specification.
Almost all schools cover the science topic Reproduction, in year 7. This is the biology of male/female reproductive systems, menstruation, pregnancy. A brief mention of contraception.
Then, contraception, the hormonal controls of the menstrual cycle will be taught in about year 10, in a biology lesson.
Students will also have some sex ed (contraception, safe sex, relationships) in year 8 and 10, through their PSHE lessons.
As a science teacher, I am often called to take PSHE classes (I think theu trust us to be pretty straight talking). So far, I have not come across any truly hideous gender ideology, although I sometimes correct terminology, and I will always answer as a science teacher, and tell the students that.
However, this dramatically depends on your school.
Also, unless you are a physics teacher, you will most likely have to teach bio and chem to GCSE. I teach all 3, and then chem at A-level.
I do always get trans questions at year 7, often stating that you can have an op and you completely change sex, that someone born male can become pregnant and have a baby if they have this op, and conflating DSDs as truly intersex.
I always tell my classes what the long established scientific evidence shows - humans are sexually dimorphic, with bodies based around the production of large, mobile gametes and gestation, or around the formation of small, motile gametes (in child friendly language).
If asked about gender, I simply say that some people believe that have a gender and that it describes their character/likes/dislikes, but that there is little scientific evidence to prove that it exists, much like many religions.