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Not only am I an attorney, but throughout college and law school I served as a member, chief justice and chief prosecutor of the honor boards so I am familiar with the process and how they think. Filing a false doctor's note as an excuse for missing an assignment, as you are seeing can be serious, BUT one way to minimize any damage here is to now get proof from your own doctor that you were indeed sick on that particular day and that this was a legitimate illness. While this note from your doctor will not absolve you of writing the fake note, it would certainly mitigate the type of sanction likely to a verbal warning/reprimand or written warning (which can be removed from your file after a year and does not remain on your record).
If you cannot get your doctor to document you truly were sick, then this is where the problem comes in and likely the sanction for this is a letter of reprimand which would remain in your academic records, since this is a first offense I am presuming.
The honor board will look at whether or not you were just taking a foolish short cut and really were sick to lighten any sanction against you (of course you are likely to get a zero on the assignment as well as that is the professor's right).
I am afraid in this type of case it is really all you can do other than apologize and ask them for mercy, as there is no real legal defense to what you stated above you actually did.
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