Interest begins to accrue on the federal portion of your student loan the day after your interest-free period—or grace period—ends. Payments are not required until you've been out of school for 6 months.
If you return to school with a full-time student loan at a designated post-secondary institution before this grace period ends, the interest that had been building since your last study end-date will be waived and you will be in interest-free status for your new study term.
If you return full-time to school at a designated post-secondary institution, but do not plan on applying for full time loans, you will need to apply for interest-free status through your loan provider in order for your previous loans to be in interest-free status.
Being in interest-free status means you are in a "funded term" whether you have new loans or not, so if you withdraw or have an unsuccessful term, it may affect your eligibility for further assistance.