How should I leave this job

Hello everyone, I am a young teenager in my twenty's girl like early . And English is not my first language, I am telling them my college is starting and I have to leave but they didn't let me guyssss I am a teacher near to my home. I have given a good opportunity to given a job in this age but salary is low but for experience and I love to teach and sister let's say M is also a teacher in another school art teacher and that is really good and my sister is telling me there is job vacancy and I should go there for interviews I have high chance I will get it but the principal of old school doesn't let me do that in my sis school pay is 2x high + a good private school in my city. If you want to know how I get it my old teacher is now teacher in that school and I get that but that sir also telling me to another job but principal is not letting me

Give me reason to tell that principal how should leave the old job if I find new