What if don't reach my 30 points for high school ?
Would that mean that I would have to stay behind for another semester and complete it ?
Therefore, I wouldn't be able to graduate from high school ?

If you don't complete the academic credits required for a diploma, then you may be required to complete your curriculum in summer school. Often, when a student is just a few credits shy, they will be allowed to graduate with their class, but be required to complete a course in summer school before getting their diploma.

If there are a lot of credits shy, then they may be required to complete another semester. But this is something your guidance counselor should be going over with you. They can best explain what you need and what the school policy is.