1. It makes them scarf down their food at an unhealthy rate
2. Many kids will decide just not to eat which will be detrimental for their health
3. This means that teachers also only have 20 minutes to eat AND prepare for their next class
4. Because kids are eating so fast they could choke
5. Many kids use longer lunches to work on assignments and go to the library, and they can't do that with a short lunch
6. With all those kids trying to get lunch at the same time, and with such a limit, it creates a stressful and hurried environment for students who basically have to race to eat
7. In truth these students only get 5 minutes to eat, with the line taking 10 minutes and finding a seat taking 3, socializing also takes up time, so kids with a 20 minute lunch probably get between 1 and 5 minutes to actually eat their whole lunch.