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 apologize for bullying a classmate, use this factual base: name the assignment, rule, person, or shared work affected and do not conceal the extent of the problem.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Ask which repair is permitted, complete your own work, and accept a fair academic consequence. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not pressure a teacher or classmate to erase a consequence or support a false account. Report bullying, threats, abuse, and safeguarding concerns to a trusted adult or official school channel. You do not need the other person to agree with a boundary for you to follow it consistently.