Boil a pot of water, hold the envelope in the steam to apply moisture and heat to the glue seal. With care, you should be able to open it, then seal it again using something like a glue stick or other adhesive. It won't pass CSI, but your parents should accept your new grades assuming you didn't give yourself all A's.
Suggestion, practice on other envelopes first. Ones that have been glued for days, not ones you licked the same day you tried practicing. Junk mail has to be good for SOMETHING.