I can't be upset about this at all. Not everyone has a mother. This isn't about 'pregnant people'. It's about the fact that not everyone has a mother in their life.
Dh and I both lost our dads young. As an adult looking back, I can totally see that we just didn't easily fit into a category with two living parents in our lives so it was hard to fit us in a neat box to celebrate all these things and I can't blame teachers or any other adults for carrying on with the status quo.
But as a young person, I found it incredibly painful to be reminded of all I'd lost once a year. As an adult, I don't think about my dad on Father's day anymore, and anyway I'm focussed on dh and the dc on that day. But as a child, it was really hard to not really be able to participate in something because you were unlucky enough to have a dead dad. I didn't have any other male family members who could have stepped into that role for that day.
Realistically, for 'mother's day' events, mothers - even if they exist - can't always attend. Our school did a 'mother's, grandmas, aunties and other special persons' tea. And there were several grannies and aunts in attendance because, unlike me in a senior role with a lot of flexibility, they couldn't just disappear from their shifts at the supermarket or on the farm to attend an event in the middle of the day. Totally fine to me. It takes a village and we should support children from all kinds of families.