I'm applying for jobs that say they offer flexible working but I need specific hours (school drop-off at 9, pick-up at 3:15). Should I bring this up in the cover letter or wait until interview stage? I'm worried about being screened out early.
I'd wait until you have an interview offer or ideally a job offer. The cover letter stage is about getting in the room — once they like you, the conversation about hours is much easier. Raising requirements too early gives a reason to filter you out before they know how good you are.
That said, if the job posting explicitly says the hours are fixed (e.g. '9-5 in office required'), it's worth either clarifying before applying or addressing it directly in a covering letter so neither of you wastes time.
I've always found it works best to raise at offer stage and frame it as 'I want to confirm how we'd make this work in practice' rather than 'I need to leave at 3pm'. It feels collaborative rather than demanding.