Despite their developing penchant for adult language and behavior, you are working with children who are going through a difficult time, physically, mentally and emotionally. Add to that the fact that students' tendency to act out may be greater in this class than others, as music's emotional and physical aspects raise their energy levels. Be firm and expect good behavior, but don't blow your top or think less of yourself if your students misbehave. Additionally, if you can conduct that heightened energy into the musical repertoire, you've won half the battle.
Again, remember that you are working with children. They are not classically trained musicians, and as choir programs tend to start in middle school rather than elementary school, a vast majority of them have never taken a music lesson before now. This is their first experience in a musical ensemble. Don't ruin it by getting upset when a single chord is out of tune or when your baritone's voice cracks. Correct, if possible, and move on with a smile.
Every middle school choir will have its fair share of less talented students. Placing these students next to or between more talented students will maximize your group's potential and give your non-Sinatra kids an example to follow.
It will always be easier to work constructively with five 12-year-olds rather than 50 of them. Especially when you're approaching a performance, try breaking your choir into groups in order to focus more personal attention on perfecting each student's work and troubleshooting. For a better result, spread your more talented singers throughout the groups so that less experienced performers might improve by their example.
Due to the many physical, mental and emotional pressures of being 11 to 13 years old, students this age can be temperamental, and may not have yet developed the ability to take criticism with poise. Always pair constructive criticism with praise. For example, say, "The dynamics of that phrase were excellent! Now, let's work on your diction a little bit."