Ever since President Obama's emphasized universal early childhood education in his State of the Union address, the chattering class has gleefully proclaimed its potential to be a game-changer for America. There is no doubt early childhood education leads to short-term academic success and perhaps long-term student sociability - but the same studies cited by advocates show that universal preschool is no magic path to help a student navigate 13 years of substandard K-12 education.

The president mentioned a cost-benefit analysis showing that every $1 spent on quality preschool corresponds to a $7 return on investment. What he failed to mention is the analysis relied on the study of the experiences 50 years ago of students at the Perry Pre-School Project, who received 2.5 hours of preschool every weekday coupled with a 90-minute weekly home visit. The amount of money spent per pupil would today amount to more than $17,000 per student.

Though comparisons on quality cannot be made on cost alone, a quick look at the cost of private preschools shows tuition ranges from between $4,000 and $13,000 nationally. Head Start spends about $7,300 per child (including its full-day program).

Ardent advocates of Head Start also cite a study showing participation decreases incarceration rates for blacks and correlates to higher college acceptance rates. However, they neglect to acknowledge a 2012 study completed by the U.S. Department of Health and Human Services that shows immediate short-term gains for participants but, "by the end of third grade, there were very few impacts found for either cohort in any of the four domains of cognitive, social-emotional, health and parenting practices." If universal preschool will be quality preschool, will it look like Head Start or will it look like the Perry Pre-School Project? The White House has hinted at what type of quality programs a future law will help fund. These include: trained teachers paid comparably to K-12 staff, small class sizes, rigorous curriculum and state-level standards for early learning. Much like we see in the K-12 education system, such reforms are at the whim of state budgets and can be prone to poor implementation and disappointment on the part of parents and students. The same wide variation in the quality of K-12 education could manifest itself in a universal preschool program.

Which poses the even larger question: Does one year in early childhood education make up for 13 years enrolled in a failing school district? Even if we were to ensure the quality of our preschool programs, the maintenance of those gains can be ensured only with quality K-12 education. K-12 education suffers from inequality among school districts, a lack of qualified teachers, and curriculum that does not make students competitive in the global economy.

Look no further for proof than the California State University system, in which at least 50 percent of entering undergraduates are required to take remedial courses in mathematics and/or writing, yet managed to have a "B" grade-point average in high school. This is a problem preschool cannot solve.

Clearly, there is need for universal preschool. Any child receiving any form of education and care in a safe environment is a net benefit to society. Lower incarceration rates, in themselves, are enough reason to make such an investment. However, the devil is in the details, and these benefits should not be overstated. If advocates of this reform over-promise, then they run the risk of thwarting universal access to preschool by future generations.

Sean LaGuardia taught math at Gilroy High School. He is a master's candidate at the Goldman School of Public Policy at UC Berkeley. This op-ed was originally published in the SF Chronicle.