The author's argument is that schools should not shy away from serving more nutritious meals to students even if they cost more to make. This argument is not stated clearly in the given passage, but can be inferred from the statement that school lunch sales did not drop when more nutritious meals were served and that concerns about money are not as important as the health of the nation's youth.
The evidence that the author uses to support their claims is that school lunch sales did not drop when more nutritious meals were served. The author also implies that the health of the nation's youth is more important than concerns about money.
The believability of the evidence depends on the context and the extent of the evidence provided. Without more information, it is difficult to determine the reliability of the evidence. However, the fact that school lunch sales did not drop when more nutritious meals were served suggests that there may be some support for the author's claim.
Whether or not one agrees with the author's claims is a personal opinion. Some people may prioritize the health of youth over concerns about money, while others may argue that the financial implications should be carefully considered.