Same as Mollytwodips- I can technically arrange a day off in term time for a very special occasion but it's very rare that I'd even ask, and I'm here from before lessons start til well after they end so I can prep, tidy and be available in case of accident.
At my old school (before I became a lone tech) as senior I was always expected to be in when school was on for safety reasons- if something serious comes up (like a big chemical spill) you want the experienced person in to deal with it.
It's definitely not discriminatory to not give people special treatment for having kids- it's not like you have a disability that limits your work hours (which would legally require reasonable adjustments- and you could argue that if it's possible and reasonable to flex time and they won't that's discriminatory) or that they're refusing to flex it for giggles because of your ethnicity, gender or other protected class (including pregnancy when you're actually pregnant/on maternity but no that doesn't carry over for a full 18 years).
If you as an employee want to organise your life around being a parent, it makes sense to take a job that doesn't have requirements that clash with that.
And as an employer if it's possible to flex time for parents working a certain position they should be flexing it for everybody in that position who wants to organise their time a certain way for any reason- elder care, transport restrictions, pet care, other commitments, whatever.
(Ahem, sorry for the essay, as people may have spotted this topic is a personal peeve XD)