American 17-year-olds are ill-prepared to make informed consumer decisions, despite the fact that they are spending more money today than ever before, according to a newly released study.

Many teen-agers are short on knowledge in critical consumer areas such as personal finance, consumer protection, economics and contracts and also lack the basic problem-solving mathematic skills needed to make the simple calculations called for in consumer decision-making, states the National Assessment of Education Progress study funded by the National Institute of Education.

The survey involved detailed questioning of 4,300 17-year-olds in both public and private schools across the nation, and involved children from all socioeconomic backgrounds.

Although the study findings suggest that youngsters are better prepared in some areas than others, National Assessment director Roy Forbes said, "there is evidence that the 17-year-olds really are not well-prepared for their future consumer responsibilities."

Some of the specific findings of the report are listed below:

One-quarter of all students questioned thought that the government guaranteed a profit for private businesses.

Only 14 percent of those surveyed realized that the person likely to benefit financially during a period of inflation is a homeowner with a long-term mortgage. More than a fourth of those questioned believed incorrectly that the person most likely to benefit during such a period was someone who had loaned a large amount of money at interest rates set during the pre-inflationary period.

Well over half of those questioned said they "probably would not complain" if blue jeans they saw advertised as being on sale were not available when they went to the store placing the ad. Only 12 percent said they "definitely would complain."

Only 43 percent said they definitely would complain if cottage cheese they bought was spoiled.

Four out of ten respondents mistakenly believed that it is legal to advertise the price of an automobile and monthly installment costs without the credit terms.

Only 40 percent knew that it is legal for a consumer to keep, without obligation, unordered material sent to him or her through the mails.

Six out of ten teen-agers surveyed mistakingly thought it was illegal for supermarkets to raise prices on the days when welfare checks are issued.

Three-quarters of the respondents thought the Better Business Bureau was a government agency, while more than half thought the primary role of such bureaus was to help businesses. Two-thirds thought, incorrectly, that BBB's have legal power to resolve consumer complaints.

One-third of the respondents did not know that a bank can deduct a service charge from checking accounts.

A third of those surveyed did not know that when an out-of-town check is deposited in a bank, the depositor cannot immediately withdraw funds against that check.

Two-thirds of the respondents did not know that warranties in advertising are as binding as warranties in contracts.

Only 30 percent of those questioned knew that unit pricing means that in addition to the price for the package of goods, the price per ounce, pint or pound is also given.

One-third did not kow that renters do not pay property taxes.

A third did not know that the use of a credit card could involve interest charges, or that if a credit card is lost or stolen the card owner can be held responsible for at least some of the charges made by someone else.

In a breakdown of the responses, it was noted that students from the Northeast region of the nation performed significantly better on the mathematical questions, but about the same as the rest of the nation on other issues.

Students in the Southeast performed significantly worse than the rest of the nation in several areas of inquiry, while students in the Central region performed better than the rest of the country in almost all areas.

Males excelled in contracts, economics, energy and financial questions, but females did better in consumer protection and purchase questions.

Students in big city schools performed below average in finance, maht and consumer protection questions.

The report also noted that students who had taken a consumer education course in school did not perform significantly better than their peers who did not take such a course.

"Without better consumer education we are encouraging a population of consumer illiterates, who will have to learn on their own in the 'school of hard knocks'," said Sandra Willett, edecutive vice president of the National Consumers League, who reviewed the study.

"Seventeen-year-olds are shockingly ill-equipped to function as consumers in the marketplace," she added. CAPTION: Illustration, no caption, By Robin Jareaux - The Washington Post