SUMMER is the relative calm before students storm their local educational institutions, be those institutions public or private, geared to youngsters or adults.

BUT even now, thunderclouds are gathering: a popular movement to train adults to be their children's first teachers is rumbling in the middle distance; commercial learning centers are raining on the independent tutoring market; community colleges are building up a new front, and parents are realizing with lightning clarity that the cost of college far exceeds tuition.

TIME to break out the umbrellas.

DONNA NELSON knew she had to help her son, Lars, an eighth-grader at Wilmette Junior High School outside Chicago. Years earlier, Lars was diagnosed as learning-disabled. Although he had attended countless hours of special-education classes in a school district regarded as among America's finest, Lars continued to struggle with reading and math, transposing words, failing to comprehend what he was reading and straining to complete even the most basic numerical problems. Last December, Mrs. Nelson took matters into her own hands.

Tapping what little extra money her family had, she enrolled Lars at a Huntington Learning Center, a privately owned tutoring hall in Wilmette that is part of a larger chain known as the Huntington Learning Corporation. Twice a week for two hours at a clip, Lars received individualized instruction in basic skills, such as how to outline a reading passage and how to divide one fraction by another.

Although the outside tutoring has been costly - the Nelsons have been paying Huntington at a rate of $28 an hour, or more than $400 a month - there is a payoff: among students doing eighth-grade reading and math, Lars now ranks near the top.

''He went from being a 'C' and a 'D' student to being an 'A' student,'' Mrs. Nelson said.

Lars's story is not unusual. In the last few years, thousands of parents have taken a similar route, supplementing their children's education by enrolling them in private learning centers like Huntington, where students receive intensive, one-on-one instruction. Although most of these centers emphasize basic skills for struggling students, they are increasingly serving the enrichment market - good students seeking to be great students. The result has been a business boom, with hundreds of learning centers opening across the nation.

''The scope of the business and the size of the market has expanded significantly in recent years,'' said Gene Montgomery, president and chief executive of Sylvan Learning Corporation, the nation's largest chain, with 480 outlets in the contintental United States, Canada and Guam. But learning centers have also acquired critics. Some educators argue that because the centers do not pay instructors as much as they could earn on their own as private tutors, they do not attract the best teachers. Others argue that the centers need to take a broader approach to learning.

Sam G. Sava, executive director of the National Association of Elementary School Principals in Alexandria, Va., while over all supportive of centers, fears that the students who need them the most might be priced out of the market. ''It will widen the gap between rich and poor students,'' he said.

Because the industry is dominated by three private companies - Huntington is owned by a New Jersey husband-and-wife team, Sylvan is a subsidiary of the Kinder-Care Corporation, and the Britannica Learning Centers are owned by Encyclopedia Britannica Inc. -financial information about each business is limited. Taken together, however, the chains report having nearly 700 locations, the great majority opened in the last five years.

Assuming average monthly revenue of $20,000 per center, a figure industry members say is reasonable, the chains bill about $168 million a year. If they are able to keep roughly 20 cents of every dollar (a proportion industry members also say is on target), the centers are generating roughly $33.6 million in earnings annually.

That is big money for a business that did not exist before 1965, when the Britannica Learning Centers, then called The Reading Game, started. Tutoring then was an informal, cottage industry in which teachers, to earn extra money in their spare time, worked privately in people's homes at rates of $15 an hour or less.

Learning centers, by comparison, are primarily franchise operations, often with business people rather than educators as owners. And while center teachers still do not earn big salaries - hourly wages range from $8 to $15, depending on credentials and where they are situated - their students pay anywhere from $25 to $45 an hour depending on the location and the type of instruction involved. Comparable rates for private tutoring in large cities average $50. The Huntington Center in Wilmette is a good example. Begun four years ago, it is owned by Patricia and Peter Cowles, a former marketing specialist and attorney, respectively. When their 9-year-old son, William, who had struggled in public schools, began showing progress after receiving extra tutoring in basic skills, the couple saw a business opportunity. They envisioned opening a private tutoring hall that would focus on the basics of reading, writing and math and would steer clear of specific subjects, like social studies or geometry. When they leses heard about Huntington, based in Oradell, N.J., they decided to buy into the franchise network.