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Glue it back on, but don't glue it to the car, just glue the pieces back together. Use epoxy, not crazy glue, and as above, don't let your parents find out
Tell your parents you knocked your mirror off and you're sorry and they'll buy a new one.
If you don't want to do that, make a stop at Home Depot and ask someone what the best product would be for gluing a mirror back on, lol. I'm sure they'll have something.