If the school believed that the majority of their parents would be happy with the speaker and the content, then why would they have been so vague, and sent the email at 4pm saying opt-outs need to be by "the end of today", whenever that is?
Even if it actually turned out to be a talk that almost all the parents would be totally happy with their kids hearing, from someone they themselves would deem reputable and trustworthy, the way the school have communicated it makes it seem super shady.
It's like if you got this:
"We want to inform you that we will be hosting a guest speaker tomorrow to talk about community and belonging. This speaker is a member of the faith community and will share their experiences to help foster understanding and respect among our pupils. If you prefer to withdraw your child from this 30-minute session, kindly let us know by the end of today."
Although it's only similar if you also happened to know that there had been many instances, over the past few years, of inappropriate religious speakers from extremist and fringe groups being invited into schools to persuade children of their point of view, without parents' knowledge, and that there were increasing numbers of children around the country joining these groups.
Sure, it's probably just a nice worship leader from the local gurdwara who's part of a regional multi-faith community project, or the friendly lady vicar from the C of E church down the road which runs fully-inclusive social groups, or someone like that, talking about nice appropriate things like helping your community and respecting others' beliefs. But if it were someone like that, why wouldn't they just say so?
The vagueness, combined with the lack of notice allowing parents to check out the intended speaker and talk, is bound to provoke concern that it's someone who needs to be slipped in under the radar, like a Scientologist or a fundamentalist religious-right Christian. (Or, at least, it would provoke those concerns if we had spent years hearing reports of dodgy people being invited to give extremist religious talks in inappropriate venues without adequate information being divulged in advance.)
I've done talks in (secondary) schools on Humanism — I sent my material ahead, and I fully expected the schools to inform parents of who I was, what organisation I was representing, and what my talk would cover, well in advance. I wouldn't want parents to have been given so little information or notice on which to make a decision about allowing their kids to attend.