Did they give a reason, or just a "no"?
You didn't answer if this is going to be an every month kind of "shortfall".... that can make a big difference in the way you should approach the problem.
Are you and your teacher the only ones contributing to the "fund" ? You all might need to take a hard look at everyone's contribution to the cause.... if there are those living there that could help, by getting a job, but won't?
Helping others that need help is a good thing, but unfortunately, there ARE those that will abuse the generosity of the givers. You need to rid yourself of those types...