You can use the plan as long as you are working at a qualified school. Your eligibility may be re-verified at any time. When your Office 365 Education plan expires:

• The Office applications enter a reduced-functionality mode, which means that you can view documents, but you cannot edit or create new documents.
• Online services associated with the school email address—for example, Office Online and OneDrive—will no longer work.
• If your plan expires, you can extend your plan by re-verifying your status as an educator, or by moving to an Office 365 personal plan.