You can answer a request for an extra shift without giving a long defence of your college schedule. Be clear about whether you are declining new work or changing something you already accepted.
Those are different conversations.
For a shift you have not accepted
Thanks for asking. I can't take the extra shift on [date] because I have a college commitment.
Stop there if you have no alternative. If you do, add a specific offer:
I can do [day and time] instead, if that is useful.
Don't offer the alternative just to soften the refusal. Check it against your timetable, travel and preparation first.
Anna University's academic-course centre maintains programme-specific schedules. Your own college's notices and deadlines belong in the calendar you use to make work commitments.
If you already said yes
Contact the organiser promptly. Explain what changed, ask about the agreed change or cancellation process and confirm the response. Don't treat a message sent as proof that the shift has been reassigned.
An original correction template:
I accepted [shift/date], but my college has changed [commitment]. I need to discuss whether the arrangement can be changed. Please let me know the required process and confirm what is agreed.
Give only the detail needed. You do not need to circulate personal academic records to a whole work group.
Hustl's view of work dignity includes clear commitments on both sides. College responsibilities should be discussed honestly rather than hidden until the last moment.
Our exam-season work plan can help you anticipate difficult weeks.
Before the next scheduling conversation, write three categories: dates unavailable, dates still uncertain and dates confirmed free. Share only the firm availability as a commitment. If an uncertain date becomes free later, you can offer it then; there is no need to promise it early because the calendar currently looks empty.