Answer:
Dear Mrs/Ms/Mr ( Principal Name ) ,
Since I am a student who only wants the best for the school,my fellow students and myself included,I am politely asking for a field trip.Because I think this would be educational and fun aswell.If you are still not convinced with this letter alone,maybe I can gather up the students' opinions through a petition.I'll be waiting for your response.
Sincerely , ( Student Name ) from ( Class name )