Is a secret nikah valid?

I have been involved with a young man for 2 years now.Because we are still studying,his father would not consent to us making nikah.However,I will be completing my studies very soon and cannot see any reason for us not to marry.His older brother is getting married soon,and as a result,his father says we must wait.I struggle with feelings of guilt.The boy is keen on making nikah,but does not want to defy his father.The boy's whole family is perfectly accepting of our haraam relationship.If we performed nikah with witnesses,but keep it secret from our families,would this be valid?