Hermione Granger was standing in the front of the Hog's Head, looking out upon a group of her fellow Hogwarts students and holding a sheet of parchment they'd all signed.
This group of students included Neville Longbottom, Dean Thomas, Colin Creevey, Dennis Creevey, Lee Jordan, Fred Weasley, George Weasley, Ron Weasley, Ginny Weasley, Parvati Patil, Lavender Brown, Katie Bell, Alicia Spinnet, Angelina Johnson, Michael Corner, Anthony Goldstein, Terry Boot, Padma Patil, Cho Chang, Marietta Edgecombe, Luna Lovegood, Zacharias Smith, Justin Finch Flecthly, Ernie Macmillan, Hannah Abbott, Susan Bones, and of course, Harry Potter and Hermione Granger.
Hermione had had the idea of Harry teaching students interested in learning Defense Against the Dark Arts as Umbridge's classes were beyond useless.
All they did in Umbridge's classes was read that damn ministry book. There was no practical value in Umbridge's classes and nor were they learning anything at all in Umbridge's classes.
Out of concern for shady characters, Hermione had everyone who was joining the group sign their name on a piece of parchment with the signing intended to be an oath NOT to tell anyone, especially not Delores Umbridge, what they were up to.
Hermione had just received the parchment back with all of the signatures on it.
She had jinxed the parchment so that anyone who told Delores Umbridge what they were up to would have the work "SNEAK" written across his or her face in hideous pimples.
But she had to tell them. She had to tell them now. Otherwise, the jinx would only be punitive and not preventative.
"I see everybody here has signed his or her name to the parchment, well done, " said Hermione in a very serious voice, "I told you already that if you agreed to sign the parchment then you agreed not to tell Umbridge what we are up to. But I must warn you all now of something. I have jinxed this parchment we have all signed. If anybody here runs off and tells Umbridge what we are up to, he or she will really regret and will pay a very severe price for it."
"How severe?" asked Harry curiously.
"If anybody here tells Umbridge what we are up to, I have jinxed this parchment so that he or she will have the word "SNEAK" written across his or her forehead in all caps in hideous pimples. Bear in mind that these pimples will never fade away completely should they be activated."
There was a screaming silence in the Hog's Head now and there was a very palpable tension.
As everyone looked around at each other everyone knew it.
None of them would even think of running off and telling Umbridge what they were up to.
"I wouldn't dare even if I wanted to, " thought everyone to himself or herself.
And they all knew the others were all thinking the same thing.
This is serious business and if we mess it up, we will seriously pay for it.
They had gone all in. There was no turning back now.
A/N: So. I believe Hermione should have told the group up front that she had jinxed that parchment. She made a serious tactical error in not doing so because the jinx ultimately was only punitive rather than preventative. Telling them up front would have made them seriously think twice before running off to Umbridge. This could have resulted in a Preventative effect. By not telling them she had jinxed the parchment, the jinx could not serve as a Preventative measure and served only as a punitive measure.