"There are a lot of students" is the correct phrase when you want to indicate a large quantity of students.
"Students are..." is correct when you want to describe students or make a statement about them.
Here are some examples:
* "There are a lot of students in the library today." (This indicates a large quantity of students)
* "Students are studying for their exams." (This describes what students are doing)
It's important to understand the difference between these phrases and use them appropriately.