It is the students who (be) to blame.

→ বাক্যে Relative Pronoun - "who, which, what, whom, when, what, whose, how, that" থাকলে Relative Pronoun এর পূর্বের Noun/ Pronoun অনুযায়ী verb বসে। 
→ প্রদত্ত বাক্যে who এর পূর্বে 'the students' থাকায় 'are' হবে।

Complete Sentence: It is the students who are to blame.

Example Sentence:
1. It is I who am to blame.
2. It is Karim who is to blame.