ALSO POSSIBLE: Half A Year LeftRemaining

Today, it became oOne hundred eighty days leftremain until the common exam for entering universityI set for the common university entrance exam.

Since starting this summer vacation, I have spent three days studying, but I(I) am concerned whether I will be able to prepare in time.

NOTE: The 2nd "I" should be deleted to make the sentence more succinct.

I must go to school during this vacation because I belong toam in a sports festival team and must practicing it, ande more, and also take some make-up classes to relearn what I didn’t understand.

Today, it becamthere are one hundred eighty days left until the common exam for entering university.

You might also say "As of today, ..."

Since starting this summer vacation, I have spent three days studying, but I am concerned whether I will be able to prepare in time.

might be better to say "I am concerned about whether"
I think "concerned whether" is okay sometimes, but it feels unnatural here.

I must go to school during this vacation because I belong to a sports festival team and practicing it, andwe have to practice, and I have to take some make-up classes to relearn what I didn’t understand.

By "practicing it," do you mean that you have to practice for the sports festival? I made the correction that I thought made sense for that, but if you meant something else you might want to change the sentence in another way.

I have many tasks to do.

This is fine, but it is more common to say something like "I have a lot to do."