You should stress the positive aspects of the job, i.e. the aspects which attracted you to apply for this job.
Do not mention the negative aspects of your current job or the job that you are applying for. Never say that you hate your boss or other staff members in that school, therefore you are looking for better opportunities elsewhere...
Also, never say that you want this job because you want to learn... If you want to learn, you should go to study and pay tuition fee... But if you are applying for a position, you are expected to possess certain skills that are needed for that position. You will get paid for what you do. Instead of 'I want to learn' you should probably say 'I am looking for improvement opportunity.'
You can mention such possible reasons as improvement opportunity, creativity, team work opportunities, etc.