I'm 19 years old and I am very thin.. it seems like for me I am the thinnest girl in our school. I got so conscious because my dress, shirts and pants are mostly of small sizes or kids size actually and I think that I looked like I am still an elementary student.
I usually sleep late because of studying. How can I gain weight faster?

How about trying some protein?

You're the thinnest girl in school and you look like an elementary student
From those statements I can assume you don't have very strong muscles you're a late bloomer in your development and well your thin.

So gaining weight fast in the state you are in now would just cause your entire body to sag which isn't healthy at your age.

All the time people think if they get thinner or get fatter they will look better which is wrong. They need to get healthier.

Eat protein have some extra eggs get some more meat stay healthy keep your metabolism fast and you'll look great in the end.