Since you have already begun, it's hard to rein in the kids. The best way is to have a class meeting. Usually Monday morning is best. Have the boys and girls sit down and ask them if they know what respect is? Get them to talk about it and then define it for yourself. Also, discuss how they show respect to each other. (When they are noisy, "messing around", they are bothering the boys and girls who are working.) You can use a T-chart with "Looks Like" & "Sounds Like" headings. Elicit that students who respect the teacher, school, and each other usually find out that they will get their work done and have time for fun things, such as as extra recess, popcorn party, etc. Go over specifics, good listener, how to line up, etc. Use a group reward and individual rewards, consequences. Put the word RESPECT on the board in bubble letters. Everytime the group does a good job lining up, etc. give specific praise and color in a letter. If they do not do a good job, then it's "Oops, do over". If individuals are having difficulty, have consequences ready...maybe they need to sit out recess and think about how it looks, etc.
Make sure you give consequences right away and make sure that they see the benefits, like, "Oh good, we have time for an extra story or game!" Or "I'm sorry, since you didn't follow directions, go lay you head down for 5 minutes or until you are ready to show respect." Think through every possibility that has happened so far and have a response ready.
Also, at the end of the day, you can pass out one or two "Super Workers" tickets to those you have caught doing a Super job. Keep track, because more kids will want that recognition!
Good luck! Student teaching and your first year are the trickiest!