This does seem like a very limiting assignment, but perhaps you're working on rhetorical questions right now and need to practice.  Rhetorical questions are kind of a tricky thing, since you risk alienating some of your audience if the question isn't just right.  For this topic, how about appealing to potential sponsors through humor--"No one here wants to see kids get in trouble, right?"  Or perhaps an appeal to the cause--"If students are willing to work and earn the privilege of a trip, would you be willing to support them?"  It's a little awkward to think of a speech given only to teachers who are also potential sponsors, but any kind of intro will work as long as the rest of the presentation is effective.  A great intro just makes your job easier.

I would wonder if rhetorical questions would be as effective than simply telling the sponsors what is needed and how the trip would benefit the student body.  It seems that rhetorical questions are good, if placed properly, but might not substitute for the actual discussion of values and importance of desires and ends.  For example, instead of asking a rhetorical question such as, "Doesn't this sound like an opportunity that could benefit students and change their lives for the better," I think it might be effective to simply go about and explain the benefits of the trip and how it can benefit many of the students.  Instead of asking, "Isn't this the composition of how memories are made," simply explain how students will be able to look back on this trip with a sense of fondness or reverie.  Obviously, the rhetorical questions featured here are a starting point for what kinds of thoughts could be posed, but I simply feel that identifying the detail behind them might be more effective and persuasive to  sponsors than raising rhetorical questions.