I had an interview here where I was asked about family/marital status/children, etc. I was never asked these things at any US interviews, but they can here, and they do.
I have been an interviewer considering whether to employ a young lady like you, Leah, and I had to try to establish whether my newly-wed applicant was looking for a permanent job, or just looking for something that was going to give a few months work until a family arrived.
At that time, I wasn't supposed to ask any question that could appear to be gender-biased .... nor take gender into account. And yet ... I had to establish the suitability of the candidate for the job. In a small office (sometimes just two staff in the building) I also had to consider things like "what if there is one of each gender"
I think you ARE still allowed to advertise for a FATHER Christmas and only interview males. The rules are less stringent for very small businesses and for personal service type jobs. Don't know about ethnic background - whether there are any exceptions in that area; as an interviewer I really don't care because I wouldn't want to take this into any sort of account.
Age - you're free to ask. School - you're free to ask. Ability to work legally - the interviewer or process MUST ask and check that the interviewee has an NI number or is otherwise permitted to work - every company has been sent a Home Office circular showing passport stamps to accept, and those not to accept, and the document is full of threats against the company if it fails to check.
Bit of a ramble - hope it help.