After our lecture in my university(year 1),A girl forget her calculator,and as it is yr1,the hall is overcrowded,so what should i do with the calculator,

You are not up to 18 . You still do not know the use of internet .

Wait until the next class, and make the announcement or if you can't get someone that could help you do that.

Or if there's another way that you could reach the owner via a public announcement, so that she could come and get it, then do so.

It seems you already have her info that makes you know that it's a lady that owns it, so use it to identify the owner.

God be with you.