The lecturer states that school uniforms are unlikely to have positive consequence as mentioned in the passage, such as making high school affordable or eliminating source of discomfort. He says that close scrutiny of each fact reveals that none of them would happen in reality.
First, the cost of clothing won't be reduced. The passage claims that school uniform makes high school affordable because students have no chance to dress fashionably at school. However, the lecturer believes the cost won't go down for two reasons. First, uniforms are usually expensive and could be got only in certain stores. Second, students could still dress fashionable after school and sometimes the mandatory uniform policy might increase their demand for fashion clothing thus making the cost stay high. It directly contradicts what the passage indicates.
Second, eliminating source of self-consciousness has many negative effects. The passage believes by eliminating difference in clothing, students will have more chance to show their personalities. However, the lecturer casts doubt on this conclusion. He says that clothing is also a source of self-consciousness, and many students choose their clothing as a mean to present themselves rather than keeping pace with fashionable style. They know what they want to show to others, and they could hide some features which they don't like about themselves. This is another part which casts doubt on the passage.
Third, teasing and bullying are unlikely to be reduced due to this policy. The passage believes that the main reason that students are teased is that they dress differently from others. In contrast, the lecturer points out that picking on others is what teenagers do because they are in a particular stage of their life. Even if everyone dresses in the same way, they'll always find others things to pick on, such as the choice of the music and a like.
In sum, the lecturer renders the passage unconvincing as it stands.