Some more terminology nitpicking and then I promise I have something more substantive. Everyone’s pointed out that “CIS” should be lowercase and isn’t an acronym. I personally just took it as emphasis. But more importantly, it’s an adjective, not a noun. You’re not a cis. You are cis. It matters because referring to someone as a trans is generally seen as offensive. Either they’re trans or they’re a trans person.

And the language you’re looking for when talking about whether someone is “non conforming to CIS stereotypical looks” is that they’re “gender non-conforming”, which isn’t really about someone’s gender identity, but about their gender presentation. So cis people can be gender nonconforming and trans people can present as gender conforming. In fact, most trans people I know present pretty in line with a binary gender. Although, it’s been my experience that this is less common with teens in general, both cis and trans, and I know trans self-identification is higher among teens.

So I think in addition to all this, just minimizing gendered language with your students in general will be helpful. From experience, it involves a lot of thinking about the intention of what you’re saying and replacing the shorthand with the intention. So instead of “excuse me, miss”, you can say, “excuse me, I need some help” or “excuse me, this is silent time”. If someone isn’t responding, “can you get her attention?” could be replaced with “a tap on the shoulder would be appreciated”. It can be hard and not always immediately doable, but the habit is more important than any individual instance.

There’s also the issue of what to do when you do misgender someone. It’s common for people to panic and sputter and just generally not know what to do, which can be worse than the misgendering itself. Point of reference, it’s like forgetting someone’s name. It’s rude, but you apologize and move on. That’s all.

if you type cis in all caps it makes it sound like you're referring to the group lead by the treacherous count dooku and not people who feel comfortable as their agab

also repeating what others say. try describing clothing, location, some physical attributes

When I substitute taught I went off what said student was wearing 'ex you in the yellow shirt' assuming there's no uniform, if there was then I would tap their desk or just walk up to them and get attention. Usually direct is best anyway. I hated when the roster would be outdated so you could end up accidentally deadnaming students and get everyone off to a bad start, going by lastname helped but it might set a weird tone.

First off, fyi, cis isn't an acronym.

This can honestly be kinda hard because there aren't a lot of nongendered terms for this, and those that do exist would definitely feel othering in this context (in the same way that being the only person asked your pronouns in like a larger introductory context is othering). My thought would be to just use some (neutral! Be careful about that part!) physical descriptions, like "excuse me, student with the black hair". Otherwise, maybe you can make a loud noise (e.g. drop a textbook on your desk) and then get that person's attention specifically when they (and others) look? Otherwise physically go over and tap their shoulder.

None of those solutions are perfect and all of them are obviously contextual, but it's what I got

i know one way

What about “excuse me, you in the red hoodie there”? Something describing clothing, hair color, etc?

"hey kid" in your oldest grandpa voice

Only one way to address unknown children

Well, you could use second person pronouns (eg. you/your) or use a noun like "youth"...

Idk, I'm out of ideas...