@Ene

Ummmm... what should I say.....

That word came from two meanings.

1. other name of 小学校.
we call that name to private school (especially talking about integrated education of private school)

2.

1. 小学校の別称。特に一貫教育の私立学校でよく使われる言葉。
2. 法的には「小学部」といい、特別支援学校の初等教育を行う課程のこと。

We call that name "小学部＜しょうがくぶ＞" in legally, that word is talking about do process of elementary education in the special class.

https://trans.hiragana.jp/ruby/https://kotobank.jp/word/%E5%88%9D%E7%AD%89%E7%A7%91-2051123

I don't have confidence that explanation,but i hope you understand ya.