When the amount equal to 612 gram silver was saved in your account then you became owner of nisab. You will have to pay 2.5% as zakah on the said amount after completion one lunar year. Likewise, next year when the year completes as per lunar calendar then you will pay the zakah of the amount available into your account. When you are the owner of the amount available into your account then zakah became fardh on you. It shall not be exempted merely on account of being a student. However, if you were minor then zakah would not have become fardh on you.
Allah knows Best!
Darul Uloom Deoband