Letter of Appointment = Teaching Contract???

Hello atty. I would like to ask some legal advice regarding my problem. Im a probationary teacher in a private school. this is my second year of teaching in this school. before the start of this school yr. we signed a letter of appointment. this 2nd semester i decided to add more subjects for my grad school and filed a resignation letter last nov 3... effective dec 2 (30 calendar days). my principal and hr officer said that the letter of appointment is our contract and i cannot resign from teaching in the middle of the school yr because my prc licensed can be revoked. they also said that teachers are not under the labor code and 30 days is not enough to find a replacement.

-are teachers not covered by the labor code?

-is the letter of appointment an employment contract? if so, why are there no terms and conditions about
resignation stipulated? (nakalagay lang po na magwork ako from june 2011 to march 2012, wala rin po nakalagay na job description at saka salary d rin nakalagay)

-am i liable for some damages? what are those?

-will my prc license be revoked?

thank u po in advance

pls help me

is this grade, high school? college?

gradeschool po

panu po ang proces ng isang guro sa pinas para makapag aply dito bilng isang teacher din sa englisg in elementary level tnx po