One grace period per loan, and this is the least known and most consequential rule on the subject. Both 34 CFR 685.207(b)(2)(i), for subsidized loans, and 685.207(c)(2)(i), for unsubsidized, provide that when a borrower ceases to be enrolled on at least a half-time basis, "a six-month grace period begins, unless the grace period has been previously exhausted". A borrower who leaves school, runs the six months out, then re-enrolls and later leaves again does not get a second one: the second departure puts the loan straight into repayment. What re-enrollment does provide is an in-school deferment under 34 CFR 685.204(b), which the Department processes automatically when it receives enrollment information from the school under (b)(2)(ii) to (iv), and which the borrower may cancel under (b)(3)(i) if they would rather keep paying. That is the right tool for the situation, but it is a deferment rather than a fresh grace period, and the distinction matters because the final departure will have no cushion in front of it.
What interest does during the six months, by loan type. On a subsidized loan the borrower "is not obligated to pay interest ... during grace periods" under 685.207(b)(3)(ii), so the loan enters repayment at the amount borrowed. There is one cohort exception, and it still affects people repaying today: 685.207(b)(3)(iii) provides that for a Direct Subsidized Loan "for which the first disbursement is made on or after July 1, 2012 and before July 1, 2014, a borrower is responsible for the interest that accrues during the grace period". On an unsubsidized loan the borrower is responsible throughout, and 685.207(c)(3) adds that interest which accrues "may be capitalized or paid by the borrower", so an unsubsidized borrower who does nothing enters repayment owing more than they borrowed. During the grace period itself neither type requires principal payments, under (b)(2)(iii) and (c)(2)(iii).
PLUS loans have no grace period, and what they have instead is not as good. Under 34 CFR 685.207(d), "the repayment period for a Direct PLUS Loan begins on the day the loan is fully disbursed", and interest begins to accrue on the day of first disbursement. The regulation supplies a substitute at 685.204(c)(1)(i), under which a student PLUS borrower "is eligible for a deferment on a Direct PLUS Loan first disbursed on or after July 1, 2008 during the six-month period that begins on the day after the student ceases to be enrolled on at least a half-time basis", and (c)(1)(ii) rolls that period into an in-school deferment the Department has already granted. A parent PLUS borrower may request the same six months under (c)(2)(ii), and separately request a deferment while the student is enrolled under (c)(2)(i). The difference from a grace period is the interest treatment: 685.204(a)(2) provides that for a PLUS borrower in deferment, "interest does accrue and is capitalized or paid by the borrower". So the six months exist, they are described as a grace period by nearly everyone, and they are a deferment that adds to the balance.
Consolidating during the grace period ends it. 34 CFR 685.207(e)(1) provides that for a Direct Consolidation Loan "the repayment period ... begins and interest begins to accrue on the day the loan is made". Since consolidation pays off the underlying loans, the grace period on those loans stops with them. A borrower who consolidates in month two of grace has traded the remaining four months for an immediate repayment obligation, which is occasionally the right trade and never an accident worth making.
Two situations the regulation handles specially. A reservist called or ordered to active duty for more than 30 days "is entitled to have the active duty period excluded from the six-month grace period", including the time needed to resume enrollment at the next regular enrollment period, with any single excluded period capped at three years; and a borrower already in a grace period when called up "is entitled to a full six-month grace period upon completion of the excluded period" under 685.207(b)(2)(ii). For a correspondence student, 685.207(f) starts the clock on the earliest of the day after completing the program, the day after withdrawal as determined under 34 CFR 668.22, or 60 days after the school's own deadline for completing the program.
What the grace period is not. It is not a payment holiday a borrower can request, because it runs automatically from an enrollment fact. It is not a status a servicer grants, so there is nothing to apply for. And it is not a period in which nothing is happening: on an unsubsidized loan the balance is growing, and the last month of it is when the servicer will send the first billing statement, which is the practical reason to log into the servicer's portal early in the six months rather than late.