Is the aim to teach functional programming as a paradigm or specifically "functional programming languages"? For High School, I would assume the first may be more important than the latter, and you may have good benefit from teaching FP as a paradigm in a language they are taught elsewhere or might have other reasons to use. You can spend less time teaching syntax and more time focused on patterns/practices.
Which is to say, maybe start out in Typescript itself? There are a lot of great more functional paradigm-focused libraries out there in "Fantasy Land" [1] and "Static Land" [2].
A benefit to starting in Typescript as well would be that you could move to Elm or PureScript if there is time/interest and reuse many of the same libraries and/or even some of the earlier TS code/projects.
It obviously seems like a trade-off that using a language that isn't "strict" or "native" to the functional paradigm, students might be tempted to avoid learning the functional paradigm and fall back to old habits. But on the other hand, that can still be a useful escape hatch to avoid frustration, especially with High School students who might have enough other stuff on their plates that some leniency might be handy/warranted (and for High Schoolers they shouldn't have that many "old" habits to fall back on anyway). It could be good teaching praxis to start with "Why did you fall back to doing this imperatively/OO/what-have-you? Did you try X? What if you explore Y that we discussed in class?" You and the students might learn a lot from dialogs like that.
Just some thoughts. There's probably no "perfect" language, but sometimes the best language is the one people already know because that gets a lot of the basics out of the way and out of the focus.
[1] https://github.com/fantasyland/fantasy-land
[2] https://github.com/fantasyland/static-land
