"Junk Food and soda should not be sold in schools." How can I argue for this idea?
In order to argue for this statement, you should start with the premise that schools are supposed to help to create a strong society. Therefore, we expect schools to, at the least, not do anything that will harm the students or the society.
You can then argue that selling junk food and soda harms students and society. It leads to higher levels of obesity and to problems that go along with that. You should point out that students have the right to eat and drink what they want. However, you can say, the schools should not help them or encourage them to eat and drink things that are not healthy.
A school acts as "in loco parentis" and since education is required by law, the free-enterprise lassez-faire rules should not apply--the students are not free to choose--they are "captives"" in that environment. Add to that the "immediate gratification" tendency in young people, and the case is made for mature decisions regarding what is available to eat in a grade school.