How can I change my name that appears on my school registration?

I haven't really paid attention to this problem until now that i graduated out of high school. Through out my school life, my mother's maiden has been placed as my middle name. I don't have a middle name. In my country, everyone is know by their father's and mother's last name. Ex: in America i will be known as Sarah Garcia, but in my country i am known as Sarah Garcia Gonzalez (Gonzalez being my mother's last name). I have seen but not paid attention to that, my school name is Sarah Gonzalez Garcia (you see the switch). Gonzalez has been taken as my middle name which in reality its just my mother's last name. I have my name (Sarah Garcia Gonzalez) on my pport and I believe on my birth certificate. My name appears as: Sarah Gonzalez Garcia on my HS diploma. What can I do?!!! I need help please... I hope that its not too late to change it. Will this affect me if i apply to a college or a job?