The costs of reducing Utah's public school class sizes would be so prohibitive that the state needs to look for other alternatives, a University of Utah study suggests.

Utah's class sizes have been the largest in the country for several years, and the prospects for natural alleviation of the situation are not good for the foreseeable future, said the study. It was conducted by the FOCUS Project (Formulating Options to Consider for Utah Schools) of the U.'s Graduate School of Education. Sue Geary is the author.Although class-size measurements differ from study to study, Utah ranks highest in most tallies.

Despite allocations each year by the Legislature to address the problem, little has been accomplished, the report shows.

Studies regarding the effect of class size on student achievement have not produced consistent results, but the majority suggests that smaller classes contribute to better learning and more satisfaction for both teachers and students. Classes of under 20 students appear to be most effective.

It appears unlikely Utah will be able to significantly reduce class sizes over the next few years unless there is a parallel tax commitment. The number of children entering classes will continue to increase.

Given the fact that bulging classes will continue, policymakers should try to make selective reductions that would have the most beneficial effect, Geary said. Efforts should be made to put low-achieving students in the smallest classes.

The study lists several strategies for making the best use of available money, including increasing staff assignments for teachers to work with at-risk students, subject grouping, use of volunteers and paraprofessionals, team teaching and split-scheduling.

School numbers aren't encouraging

- Class size in Utah's public schools is a serious problem that will continue into the 1990s.

-Average class size in Utah per teacher is 23.6, compared with 17.9 nationally.

- To decrease class size by just one student per teacher would cost $17.7 million.

- To decrease class size to the national average would cost $159 million.

- Utah's school population will increase by 31 percent (132,000 students) by 1995.