Interest begins to accrue on your student loan the day after your interest-free period ends. Payments are not mandatory until you've been out of school for 6 months. This is known as your non-repayment period.
If you return to school with a full-time student loan at a designated post-secondary institution before your 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 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.