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If all your rules are going out the window, that means you've been lax with enforcing them and they're taking advantage of that. I suffer from this problem every year, myself. It's hard to keep to your own rules when you're a naturally friendly/non-confrontational person.

It's going to take a lot of work to get your authority back. Unfortunately, without knowing your students I can't give you any specific feedback. Also, I work in a school environment that's a lot different than the US (i.e. I can smack a kid lightly on the head with a rolled-up piece of paper if I think he's being a jackass).

My one suggestion would be, if junk food is becoming a problem in your break time despite being against written class rules that have been in place since the start of the school year, effectively ban food during breaks. Send a letter home about it. I'd suggest getting the students' addresses and mailing a letter in addition to passing one out in class so parents are aware that students have overstepped the boundaries and you are banning everyone from food. Explain that water in clear bottles is acceptable but no food whatsoever until they can show responsibility with following class rules. If they take up too much time, start docking participation points for each minute they aren't ready. Bring a stopwatch and time them down to the second and post the times on the board for the entire week so they can see the pattern of going over they have developed.

I guess I have a second suggestion: stop allowing make up tests if students refuse to listen in class. The more students feel like they can ******** off in class because they get a second chance if they ******** up on tests, the less reason they have to study for tests the first time. This is a problem my school has on a school-wide level because re-tests and make up tests are rampant in every subject. If you don't allow them, you're on par with Hitler in the students' eyes. It has allowed kids who don't particularly give a s**t to half-a** it the first time, then get the correct answers from friends to pass the make up.

Your students have to be shown that there are consequences for their actions. If they don't respect the class rules, their freedom in class will dwindle.

No matter what age your students are, you have to be firm and consistent. They'll probably whine (it's the same with my kindergartners), but tough. You make the rules. If you have preset class rules, you have to enforce them every time. Even if it's one of the good kids stepping out of line, you can't make exceptions (especially not in front of the class).

As for the snacking, I have kind of the same problem since my kids eat breakfast in class every morning. I was starting to get worried that it was taking them ~45 minutes to eat, so my principal just told me that I need to set a timer for how much longer they have (there are some websites that have a timer you can show on a projector), and when time's up, all the food has to be thrown away.