Employment

I am degree student pursuing computer engineering,this year I applied for a job as a part time in a school for teaching field and i got selected though the school asked me to submit atleast any one document of mine so i did submitted my marksheet as a proof document to the school after this ... I serve as a science teacher for 3 months in that school, after 1 month in working I applied for a degree in government college and i get selected (which took atleast 2 months for the selection session and for the result declaration)so I decided to resign the job in immediate case i put the resign letter to the principal and asked for my document(my marksheet). In reply she said no 'I won't gave you your document and im not accepting this also' . As she talked in very rude way, I decided not to come even though she rejected the resign letter. Also I have my genuine reasons. Now it's been a 3 week she haven't returned my marksheet. That's why I'm here to ask uh is there any rules like this that i can't leave the school before one year , also the school haven't alert such things when I was joining so who could do I know now please help me out