Depends on demand. Why not ask universities directly?
The university I graduated from offered it to anyone willing to pay the fee if there were still spaces left (university students obviously had priority). But that was a part-time thing for 40 credits (4 classes a week).
Don't expect to become anywhere near fluent with a university course. They're a nice start. The language school I attend right now covered the entire University of Sheffield Japanese course in about six months. ...
It might be smarter to finish high school, graduate from university, and then focus on Japanese. Have you ever been to Japan? Why not do a summer program at a language school and see if you still like it after that.
If you need something to occupy yourself with until then, why not learn hiragana and katakana? Your signature is in romaji, so I'm assuming you don't know those yet. And if that's not enough, you can learn easy kanji for numbers and basic words.
Good luck.