Not Stay Up [As a title, you should have a phrase such as "Staying Up Late" or "I Shouldn't Stay Up Late" or "I Should Get to Bed Earlier"; the phrase "Not Stay Up" feels like an incomplete phrase.]

Oh, I have stayed up to midnight once again [Because this is a completed action in the past, you should use the 'perfect tense': "I have stayed up..."].

I must give up this bad habit, for the reason that I may doze off in class in the daytime. [Excellent!]

Besides, It’s harmful to my health.  [Excellent!]

Girls should get their beauty sleep, but I always miss it.  [Excellent!]

Maybe this bad habit will make me old really fast, which is terrible. [Okay, but you are talking about one bad habit, which is staying up late. Therefore, I would say "this bad habit" or maybe "my bad habit".]

Oh, I really can’t imagine that, and I want to be young forever, so I have to rest regularly from today.  [Excellent!]

Dear Lili,

Thank you for correcting my entry. I hope my comments can be of help to you.

~ Ned (Na4de2 -- this computer doesn't write Chinese ^_^)