Is it your master's intention to widen your asshole? Or this buttplug task just supposed to keep you mindful of what a slut you are while sitting in class?

If it's the latter then I would recommend something like this. It's a heavy steel ball with holes to put a string through. It slides up one's ass and leaves no external trace, except for the string. The weight and girth of it, however, should definitely do the trick of keeping you focused.