School uniforms eliminate the need to keep up with fashion. Expensive clothing fads come and go, especially for students. All students wearing the same uniform all year round saves money.
When students are permitted to wear what they want to school, they become subject to the latest fashion trends. Parents are forced to keep up with these trends and buy their children the latest clothes or risk having their child get teased for looking outdated or uncool.
In this respect, school uniforms save parents money. Instead of having to buy new clothes with each incoming trend, parents only have to replace the school uniform when their child physically grows out of the clothes or they become ripped or damaged beyond repair.
Over the course of a child's school life, this saves the parent a large sum of money.
School uniforms are also cheaper than normal street clothes. A UK study found that the average school uniform costs parents £88.05. The average non-school outfit, on the other hand, sets parents back £113.00. 
How can anyone argue that forcing parents to spend money on a set of clothes the child can only wear at school is cost-effective?
While uniforms may be cheaper than non-uniform outfits, they are also an entirely unnecessary and additional expense. Families don't buy uniforms instead of street clothes (because the child needs something to wear when it is isn't at school), they buy them as well as street clothes.
In this respect, no matter how cheap uniforms are, they are still an additional expense.
The Guardian reported that 95% of parents from low-income families had difficulty meeting school-related cost. The additional expense of uniforms was a huge component of these school-related costs.
[P1] School uniforms are cheaper than street clothes.
[P2] School uniforms mean parents don't have to keep up with fashion trends.
[P3] In the interest of money, school uniforms should be mandatory.
Rejecting the premises
[Rejecting P3] School uniforms are an additional expense.