Do I have to leave Thailand within 24 hours after my work permit and visa cancellation?

Sounds super okay.

Your school gave you the 30th June as your last working day. You can apply for cancellation effective on 30th June in advance from now until 30th. And you have until the end of 30th June to leave.

You don't have 24 hours after the cancellation of your extension got effective, you only have that day. You need to leave within the last day (before the date changes at midnight).