Punctuation and sentence type.

Students ,who scored low marks in the past exam, can pass the national exam if they study regularly.

I just edited to make changes. 

Your sentence needs a comma and a period.  I would change “in the past exam” to “on a past exam.” 

If a student who scored low marks on a past exam studies regularly, he can pass the national exam.

In Sanjay’s corrected sentence, there is also a problem with the pronoun.  “He studies” does not match “students.”  One of these needs to change, to either “a student” or “they study.”

The sentence is awkward and isn’t entirely true. Studying regularly will surely help a student improve, but not every student will pass an exam.  There will always be some people who cannot master a subject, no matter how hard they try to understand it. 

In spite of a low score on a past exam, a student may still pass the national exam after regularly studying.