Here are some good general principles:

1. Have rules, and post them in your classroom. Go over them with the students so they understand them.

2. Have consequences for poor choices. It can be as simple as names on the board and checks for subsequent rule violations. Be strict and enforce the rules consistently. My system is names and checks: the kids get their names on the board as a warning (shape up!) and the first check is a report to the classroom teacher that I had an issue with the individual's behavior (sometimes there are more names than one: that's okay!). For middle school the first check is I contact their parents and after class conference, 2 adds a detention to the first, and 3 is 1 and 2 but I remove them from class as well. If an individual gets three checks in a class period, they go to the principals office. The checks reset each class period unless I have frequent flyers.

3. Have rewards for good choices. In my student teaching, we had the "prize box," which had various things in it, but mostly candy. Each quarter, the students could choose something from the prize box if they did not have x number of bad reports. For the older kids, such as your middle school students, you could see if the school will support (and pay for) a pizza party or something like that for each student that keeps their nose clean and gets 100% in your class (stress no rule violations and perfect class score).

4. KNOW THEIR NAMES! Nothing strikes fear into the heart of a student more than hearing their name in a pointed manner when they know they're misbehaving.

5. Be confident and be yourself, kids can smell fear and a fake from a mile away. The two go hand in hand, as being your natural self in front of the class will help your confidence.

6. Over plan. Having something for the students to do for the whole period and keeping them engaged will mitigate the opportunity for misbehavior.

You can ask some other teachers there what they do. Acting like a teacher will garner their respect: certain mannerisms will tip them off that you're no amateur and they won't pull some of the more basic shenanigans. If you want more specific tips, PM me.

good , all of the above.
my schola is 6-9 yr olds, not sure of the American system for ages.

main thing, keep them busy, move lightly from one part to another, so nothing gets bored or stale to quickly.
Watch out for students who are

a) slower than average. If they get left behind they may act up to hide their embarrassment at being lost. Break the topic down more. Make sure you are isolating and teaching elements one at a time (as Ward does). Can you add more cues - ie visual and aural etc. See what helps them over time and big it up.

b) quicker than average. Often neglected, these children can cause problems because of boredom. Have 'extra' tasks for them to complete while you go over the material more slowly with others, otherwise boredom may induce messing.
for example, I had one boy in a catechetics class who was really a bit of a genius (getting extra tutoring for gifted children etc.). I was going through the idea of the liturgical calendar with the group, and as I could see he had got it and was getting shifty, I asked him to calculate how many hours , and seconds until next Easter (as mental arithmetic). That gave him a few minutes on the sort of puzzle he loves, while I could finish up with the others. Granted that was not a very useful thing to ask him to do, but it was on the fly, if planning to have him regularly, I would always be bringing to class some little extras for him.
One boy in my schola who has limited attention span, I allow to colour. (I give him bits and pieces from the nice religious black and white artwork posted on CMAA, they make great complex and absorbing colouring pictures.) I know he is listening, because he answers the questions I ask and always stops to sing, but the colouring is enough to stop the fidgeting which would lead him astray. And he has a beautiful schola folder at the end. Over time the amount I demand from him has slowly grown, so he spends less and less time colouring and his attention span has improved.

As you are teaching intervals to sing, build them into warm ups to get the sound into their ears. So if we are doing two note gaps in class, we will have some warm up which reflects this. I find this works, as the are primed with the sound we are then analysing and practising.

Decide yourself who sits where. Put those who need it for whatever reason near you, break up friends who might encourage each other to mess etc. This also helps you to remember names in a big class, because you can have a chart in front of you with their names according to where they are sitting.

if the room is big enough for the size of group, have a singing space and a working/ desk space, so that you can move from one to the other.

Pray with them before hand if it is allowed. If it isn't sing a prayer as part of your warm up. Pray over them silently to yourself before you start.