http://urbanlegends.about.com/library/bl_certificate_of_completion.htmIt does not affect future eligibity for diploma or GED
Does being a recipient of a certificate of completion render a student ineligible to receive a diploma or GED at a later date?
No — although, again, the law varies from state to state, and there may be time limits or other constraints affecting a given student's continuing eligibility to receive a diploma or equivalent.
In Indiana, for example (home of the "ISTEP" test mentioned in the email), a student who has received a certificate of completion can still elect to return to high school and complete the requirements for a diploma (or enter a GED program) through age 21.
It is not a permanent obstacle to attending college, etc.
Does receiving a certificate of completion or attendance mean you can never, as long as you live, join the armed services, attend college or trade school, or get a federal loan?
No, that's complete rubbish.
Nearly all colleges and most trade schools do require a high school diploma or its equivalent for entrance, so anyone holding a certificate of completion will need to go back and complete the necessary academic requirements to get one before they can apply. The same is true, generally speaking, for military enlistment and receiving federal student aid. But there is no sense in which the mere fact of having received a certificate of completion or attendance permanently disqualifies one from pursuing any of these goals.