All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His Slave and Messenger.
The fact that the credit card which you mentioned is exempt from interest during a certain period, but its holder is obliged to pay interest after the expiration of the grace period, then this does not make it lawful to deal with it as the condition of Riba (usurious interest) still exists. Accordingly, it is forbidden to enter a prohibited contract without a necessity, and the purpose of decreasing the interest of your student loan is not considered a necessity that allows entering into such a prohibited transaction. Also, this is not a solution to the problem; rather, this may aggravate it. The only solution is in being committed to do what is lawful and avoid what is forbidden and repent from it.
The decision of the International Islamic Fiqh Academy No.108 (2/12) regarding credit cards states: “It is not permissible to issue a non-covered credit card or to use it if it is stipulated in it that Riba interest is to be paid, even if the one who requests the card is determined to pay it off within the free grace period.”
Besides, the two Fataawa which we referred you to, 88473 and 98959, clarify the prohibition of entering into any usurious contract in principle, even if it is with the intention of repaying before the maturity date of the interest rates. One of these two Fataawa gave an example of a bank that gives a car loan with a grace period of 36 months on the condition that the bank deserves the interest thereafter. This is similar to the contract of the credit card that you mentioned in terms that the contract includes a grace period from interest and then it demands the card holder to pay interest after the expiry of that period; so it is a forbidden Riba-based contract.
Allaah Knows best.