Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to tell your parents you are failing a class, use this factual base: include the course, assignment, policy, dates, documented barrier, and result you are asking to review.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Request the permitted remedy or meeting and be ready to complete the required work. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not demand a grade, conceal misconduct, or ask another student to support an inaccurate story. Use safeguarding, disability, counseling, or emergency channels for bullying, abuse, health crises, or immediate risk. You do not need the other person to agree with a boundary for you to follow it consistently.