皆さん こ___にちは

初めまして
私は高校生です
今十七歳です
私は日本語が少しわかります

皆さん こんにちは

初めまして.
私は高校生です.
今17歳です.
私は日本語が少し分かります.

Explanation:

Red: Kanji confusion.

Blue: Missing syllable causes confusion. Without the extra n (ん) 皆さん こんにちは goes from (Hello everyone) to 皆さん こにちは (This is blood everyone [translation is a little rough])

Orange: Numbers written left-to-right (not top-to-bottom) are written in English/Arabic numbers, not Japanese. Japanese numbers are for top-to-bottom.

Translation: Hello everyone!

Nice to meet you.

I am a High School Student.

(Right now,) I am 17 years old.

I know a little Japanese.

Good work. Minor problems, but still good.