about marriage

need little a help 
i want to marry a girl she doesnt belongs our caste her age is 17 years 3months and i am 23year old.
but according to our law we can register a marriage when girl is 18 and boy is 21

we both dont have much time we want to get marry soon we cant wait for me to be 21
there is  problem from my family side and  her family dont know  about our affair and will probably going to take steps if we conduct marriage in temple   which is not legally authorized because her age is still now 17 years 3 months so  please suggest what to do next.how to get marriaed as soon as possible by registerring in court or any mean plzzzzzz suggest how to get married soon plzzzzzzzzzzzzzz reply soon