If a person is gay, lesbian, or trans, they will know,

I'm not sure that is the case. Two of my cohort didn't come out until they were much, much older. One came out as gay in his thirties, the other transitioned in her forties. Both experimented with cis relationships. Both found what they were looking for elsewhere. It took decades to figure that out.

so why do they need to be educated by someone who likely knows less than them?

I think it takes some people a while to figure things out. Especially if it means doing something outside their experience.

I would think that letting kids know about other sexualities early would help them understand why they aren't interested in what their peers are up to.

the latest education I received regarding sexual education was in fifth grade,

I'm closer to the class of 2000 - I'm surprised that the last round of sex ed you received was only grade 5. I'm pretty sure we got a rundown on straight sex (focusing on STDs/pregnancy/condoms) and a warning about gay sex (AIDS! srsly) in grade five or six, and then a reminder in grade 8 or 9.

There was no follow up, there was no peer to peer discussion, only adults telling us what the world was.

What kind of follow-up would have made sense?

we shouldn’t teach people all about sex and even sexual identities, only that we should teach them later in life, when people feel more confident in standing up to authority, and thinking for themselves

What kind of standing up are you referring to? When I hear "standing up", I think of people protecting something. Is that what you mean?