If this is your first year teaching, you should be now leaving what teachers like to call "the honeymoon period" and kids are comfortable with you and will try to test you, if they have not already done so!
Some things to keep in mind to maintain control of the class:
Students, no matter the age LOVE consistency. If you stick to the rules and procedures of your classroom and do what you say you will do if a student breaks the rules, the disruption will be kept to a minimal.
Students LOVE positive rewards and respond to that way better than consequences. Keep a bag of candy or fake money students can earn to get a prize in your desk and pass them out to students caught doing good things, doing what they are supposed to do, or simply because they did great on a test or quiz! Make sure you reward students who are often trouble-makers when you catch them doing something good, or simply just being on task or keeping quiet for a whole 10 minutes! Kids who stir up trouble are often times looking for attention and that is the only way they know how to get it. Show them that if they behave and do what they are supposed to do, they will get attention.
If you get the "ring leaders" of the classroom on your side, it will help tremendously in classroom management. Ring leaders like to be in charge, so give them something to do - assign them leader of their group, allow them to be paper passer or be the timer.
Make POSITIVE phone calls and notes to send home! Parents don't get this often and it can help establish a good working relationship with parents if you do this, so when a student misbehaves, parents are more likely to work with you.
If you have a 90 minute block, allow time for students to breathe and walk around a bit, especially those hyper active kids! Incorporate movement into your lesson (IE gallery crawl, stations)