The form CAN be submitted because the Event Registration is separate from the payment portion.
Once you click Continue or Submit to go to the Payment Screen, the Registration is essentially completed, however Payment is not until the Payment is completed and Submitted. The usual cause of this is the User didn't finish filling out the CC information or possibly clicked Back or Cancel which would have aborted the submit of the payment portion for this Event Registration and thus not returning the CC information to the form.
After confirming the Payment does not display within the Payment Gateway directly, your School will need to contact the registrant so they can attempt to register again. The reason for this is because the User probably don't know that something went wrong, and that Payment never went through for this Event Registration.