As you might know, in Japanese, an informal speech differs a lot more than a formal speech compared to English. So I'm going to correct in both ways.

bad habit during school (学校での悪い習慣）

おれ は すいみん が すこし いる。おれ の からだ が しんう だ。

私は少しねてしまいます。　体が死にそうなので。

We have a great expression "2nd form (like the stem of したい、見たい, which means し and み　is the stems here） + まう" and "そう". I can explain you more but it could be long, so i suggest you to study by yourself :)

And you can change MAU into "ちゃう", so "俺は少し寝ちゃう”　(here you need 1st form) 

and 体が死にそうだから。

Take a note that we dont repeat "MY,YOUR...." like in English.

What I'm trying to say:
I get little sleep. My body is dying.

bad habit during school　授業の間の悪い習慣

おれ は すいみん が すこし ねる。おれ の からだ が しにます。

What I'm trying to say:
I get little sleep. My body is dying.