@rahul-Here nothing more is mentioned about the joint account other than its name.
So, we cannot say whether a joint account exists between two students as a joint account can exist between a student and any other person who is not a student(maybe his family member)
But yes, the important part is
BankAccNum is UNIQUE AT BANK LEVEL- means say for two banks SBI AND PNB, there may be same account number which belongs to both banks.
But say AT SBI, BANK A/c Num say 'a1', will be unique and a student can have multiple bank accounts in different banks in which case our DB can contain same account numbers.