I tell my students from the beginning that my job is not to give them the instrument they want but to help them find the instrument they will be successful on. Sometimes it is the instrument they want and sometimes it isn't.
I also explain to them that band is like a football or baseball team. Not everyone can be the quarterback or the pitcher and we need people to cover every position. The students seem to understand this concept and I have not had any problems since I started doing it this way.
I will also say this works for me because I teach in a small school we check out all instruments.