When you make ANY payment on an interest accruing loan (there are other types but they are more rare and student loans are simple interest), the payments are applied first to charges (like a late charge) and then to accrued interest, then to principal. It’s not possible for an extra payment to ALL go to interest if you are current on the loan. It can only go to the interest that has accrued since the last payment, and a much bigger portion will go to principal.
When you have any type of loan (car,mortgage, etc) you’ll receive a amortization schedule. You have the right to follow that and as long as you pay within the grace period, you’ll be fine. Student loans are the same. My daughter can pay the scheduled $217 per month and it will work out in 10 years, but if she pays 30 days, then 15, then 45, then 30, her charted amortization will look like a roller coaster.
She read me a statement on her payment that said that if she paid more than scheduled, the ‘extra’ would go to the principal of the loan with the highest interest rate - which is exactly what we want it to do. Where kids (and adults) get screwed up is if they pay double in one month and they think they can skip the next payment. No, that amount has already been applied to principal and not ‘saved’ to pay the next payment. If the next payment is more than 30 days later, a late charge may have been assessed and more interest than the amount of the payment may have accrued, so no principal is reduced. Pay every 30 days and if you pay extra, realize you haven’t ‘paid ahead.’
The interest rate for loans is set on July 1 of each school year. The rate is different for loans taken in different years. I’ve not seen a different rate for subbed and unsubbed loans taken in the same year.