Is it permissible to use student loan funds designated to cover living expenses, on which no interest accrues as long as one is a student, to purchase some essential (non-luxury) necessities, such as halal food when feeling hungry?

There is no harm in buying food with loan money as long as it does not affect your studies. It is preferable to buy from the money allocated for living expenses. You must be keen to finish your studies and not prolong them because of the loan. If interest is requested on the loan, you are only obligated to repay the principal.