Would it be appropriate if I wrote thank you notes to all of my teachers (or at least the ones I will have again)?
Of course! Teachers love getting a heartfelt note. Most teachers aren’t doing it for the paycheck and these kind of overtures are appreciated.
Also I’m not a senior so would it still be appropriate
Yup!
I send thank-you notes to all of my teachers every year! 
It’s always appropriate to say thank you!
Why would you only write thank you notes to the teachers you’ll have again? If you’re only going to send them to a few teachers (and not all of them), it seems more logical to send them to the ones you won’t have again now, and to the others after your last class with them.