Jisho

高一 is a abbreviated way to say 高校一年生

https://jisho.org/search/%E9%AB%98%E4%B8%80

You can do this for any grade level, 小六, 中二 (that's another one with cultural implications to the abbreviated form, if you're interested in looking it up), etc.

It's just that most of them, when abbreviated, don't also sound like a common male name. They just sound like abbreviated grade years. 高一 has that distinction.

Thanks @Leebo. I was aware of that meaning. What I'd like to know is if Koichi/Kouichi is ever used as a general term of endearment for children or boys. I feel like my colleague, or the author she is editing, thinks it means something like "little one" in English, and I don't know of this usage.

Well... like... I guess if you're talking to a high school first year student, some people might enjoy using it more since it sounds like a name.

But using it when they're not a high school first year makes no sense.

Thanks for taking the time! :)