There are 30 students in my class, I've collected the height from all of them. I am trying to calculate the standard deviation based on the data. Should I consider the data sample or population?
I guess it is population std, I just need a double confirmation.
Population standard deviation (i.e. where you divide by the sample size) will tell you the standard deviation of the heights of your class considered as a whole, i.e. you don't care about the fact that people from other classes or other parts of the world might have different heights.
Sample standard deviation (where you divide by the sample size minus $1$) is best when your goal is to use these data to infer some estimation of the heights at a larger scale (and for this to work you need to be able to argue that the people from your class can reasonably be considered as a random sample from your larger population).