Hey Tino!
I’m actually in the exact same spot as you are, as I’ve got a performance for an event at my school too in May. I’ve played at it before too, so I’ll try and tell you what works for me when I do stuff like that.
Here goes! Sorry for the wall of text. 
So, given your level of playing, I think those songs are a really good showcase of the instrument. Assuming that’s currently your full repertoire, and since you can play those well as you’ve said, I’d go for all of them if you have the time.
In that concert at my own school one of the songs I’m playing is Yasaburo Bushi with a singer, and as an outro I blended in a bit of Ringo Bushi. I’ve noticed it can be kind of difficult to easily blend Ringo Bushi into the beginning of Yasaburo Bushi because of the ni no ito and san no ito “chord” on the 2, but it’s not too hard to nicely blend the end of Yasaburo Bushi into Ringo Bushi.
I like to finish out Yasaburo Bushi and on the ni no ito slide from 1 to 5 and play the section that starts like that and lead it back into how Ringo Bushi normally starts and play it out from there. If this doesn’t make sense please lemme know, because I explained that sort of weirdly. Not really sure how else to put it but if you’d like I can try.
As for the rest of your repertoire, you might be able to use the end of Sakura Sakura to blend right into the beginning of Kuroishi Yosare Bushi since Sakura ends on an open string across all three and Kuroishi Yosare Bushi starts with the same thing. If you’d like to play those in a blended set, I personally think that sounds nice. Whatever you prefer, of course!
So, about the freestyle. That’s sort of along the lines of a Tsugaru Jonkara Bushi kyokubiki (which is pretty much “trick playing,” if you weren’t already aware, it’s pretty much fancy-flying-fingers using Jonkara phrases in an improvisational version of the piece. It’s basically whatever your own Jonkara Shin Bushi is, and you can improvise how-ever).
Note: This section only applies if you want to play Tsugaru-style improv. I know next to nothing about Nagauta/Jiuta or if there’s freestyle in those genres, so you might know more about that than me.
So, if you play or are working on your own Jonkara Bushi, I’d say go right for the freestyle, because you have a feel for how that sort of thing should sound. If you play any of the Godai Tsugaru, really, you might be able to improvise around their base structures.
I’d suggest though that if you don’t know those or Rokudan yet (as Rokudan has a lot of quintessential, usable phrases in the Godai Minyo, especially Jonkara Bushi), spend some time with those and see if you can really get the feel for it. Listen to other players and maybe use Kevin’s Jonkara Bushi article here on Bachido for structure help. I would say though that it has taken me multiple months to get my Jonkara Bushi to a point where I like the way it sounds, so you might not have the time before this performance to hammer a version out. The reason it’s like this is because although it’s technically improvisational (and can be freestyled) it takes study and practice to know how to fit it all together into the actual song. It’s not just random notes, there’s a bit of an art to it. If you feel that however you’d like to improvise is ready, though, and sounds good (even if it’s not the traditional Tsugaru sort) go for it! It’s your music, and whatever you make of it.
Lastly, yep, chairs are nice (Seiza gets painful haha) and I like to position the microphone so it’s a little bit out from the dou. One time I wasn’t careful with how far out it was and I accidentally hit it a couple times, so make sure it won’t interfere with your bachi technique! Other than that, that’s probably all you need the school to supply. Unless you do want to sit seiza, in which case, maybe they have a zabuton?
And as for playing in front of an audience, and this is the most important thing here, HAVE FUN! Unless you’re a fixture at school events most people where you go probably haven’t even heard of a shamisen, so rather than feel nervous, just try to get a kick out of their reactions and enjoy what you’re playing. Those people in that audience are your friends, and they don’t want you to feel nervous. Just put on a smile, get out there, and do your best.
Sorry for the giant wall of text, again, I just have a bunch of ideas. Hope it helps! Good luck! Lemme know if you have questions.
