Forgot to add misdemeanor on app

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Grounds for rescinding an acceptance, yes. You did not disclose a misdemeanor to the school prior to them accepting you, which is grounds for rescinding an acceptance. That having been said, while it is possible and defensible from a school standpoint, I agree with @LizzyM that they would be unlikely to do so.
 
And you make the letter as brief and as just factual as possible. What age were you when you got this ticket?

Sample what to write
"I failed to note that I received a ticket for essentially jaywalking on such such a date. I did not realize that this would be considered a misdemeanor in my state and therefore neglected to note this on my application. This was an oversight on my part and was not intended in any way to mask this incident.

Thank you"

I sent a similar email and was notified by the school it was not a big deal. Huge relief, thanks!
 
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