The trumpet is the smallest member of the brass family. The
sound on trumpet is produced by buzzing into a small
mouthpiece.  Trumpet players generally need thinner lips that
can roll slightly between the teeth and fit into the mouthpiece. 
While orthodontia is somewhat troublesome at first to a trumpet
player, it is not impossible to make good sounds with braces. A
slight overbite is okay, but an underbite can severely hinder
progress on trumpet.

Trumpet checklist

* Medium to thin lips

* Minimal over bite no under bite
 

Other Considerations
Trumpet parts usually have the melody (recognizable) part,
therefore students who choose and are selected for trumpet
should exhibit a confident demeanor, strong personality, and
demonstrate a high level of self-motivation.