If location is the key to a good real estate purchase, for many Silicon Valley buyers a good school district is the key to a good location.

Real estate agents await the days each year when the state releases its Academic Performance Index, the numerical rankings that measure how well each school's students performed on standardized tests. But a high ranking might not always mean a home will gain value faster.

It's not just families with school-age children who check out school test scores before hitting the open houses.

''A lot of family members are looking for a school district for their unborn children, ''said Richard Calhoun of Creekside Realty.

Some childless buyers are simply planning ahead. But others ''just heard that it's better to buy in a good school district,'' said Jeff Hansen of Keller Williams Realty in San Jose.

Private vs. public

Whether it's always better, though, is a complicated question. Rachael Berman is discovering this as she, her husband and their 2-year-old son prepare to move back to the Bay Area after a few years in upstate New York.

''We looked at schools the last time we bought a house, even though we had no children at the time, ''said Berman, a chip designer who is starting a new job in the Bay Area in August.

This time around, she is crunching the numbers to see if they might be better off paying less for a house and using the money they save to send their son to a private school.

The result?

''I feel like it's kind of a wash, ''said Berman. If they pay more for a house in a better school district and stay there, they will have higher mortgage payments for 30 years, instead of private school tuition for just 13. Their property taxes will also be higher. On the other hand, those expenses are tax deductible, whereas private school tuition is not.

Another consideration: The private school Berman looked at includes after-school care and activities -- both items that parents pay extra for if they send their children to public schools.

Specifics matter

For buyers trying to decide whether it's worth it to pay more for a home in a highly regarded school district, every part of the calculation depends on their specific situation. And it's not easy to know exactly how much of a home's value is related to the school district.

David Blockhus, an agent with Coldwell Banker in Los Altos, estimates that buyers pay a premium of 10 percent or more to be in the Los Altos school district instead of the Mountain View district, or in the Cupertino district instead of the Sunnyvale district. A better school district often has other attributes -- larger lots, larger houses or a better neighborhood -- that also increase the property values.

For buyers with school-age children, there are several factors to weigh when deciding whether a better school district is worth the price:

• How long will you live in the house? Higher mortgage payments and property taxes could last longer than private school expenses.

• What kind of private school are you considering? Tuition varies but can exceed $20,000 per year. Admission to private schools can be competitive as well.

• How many children do you have? Berman did her calculation for one child. If she had more than one, she said, paying for private school wouldn't be an option.

• Will you have the cash available to pay for private school? Many buyers who decide to buy in a less sought-after school district don't actually spend less on their home -- they just buy a bigger house. Even if you borrow less money, that's not the same thing as having the cash to pay private school tuition.

• How important is it to live near your school? Some parents prefer to send their children to a neighborhood school so they can spend less time driving and make it easier for their children to see their friends.

For buyers who are looking at the good school district primarily as part of their housing investment, the picture is less clear. Many real estate agents and buyers say homes in sought-after school districts hold their value better and are easier to resell, and some anecdotal evidence supports this.

But Calhoun's data, for example, shows no particular connection between the most desirable school districts and appreciation rates in the past eight years. And if the schools in an underperforming district improve, a home there may even appreciate more.

''Buying in a poorer school district not only gives you more house for your money, it may be a better investment, ''Calhoun said.

A final consideration is that the test scores that most buyers use to evaluate districts aren't the only measure of a good school.

''They want a good education, ''Hansen said. ''Their end goal isn't a number.''

In the end, the choice is more than a financial one for many people.

''If you want to signal to the rest of the world that you are a serious parent when it comes to quality education, where you chose to live is one way to do that,'' said Hersh Shefrin, a Santa Clara University professor of finance who studies the psychology of financial decisions.