The prom/graduation dress is for many teen-agers the single most important purchase they make by the ripe old age of 16 or 17.

Even if a mother gets into the act and the school offers guidelines, the dress -- more and more girls are wearing the same for both occasions -- represents an independent statement of fashion attitudes.

"It is," as one mother put it, "a big psychological moment. You know your daughter has grown up. She's going to hold out -- strongly -- for her choice. dMore than ever before."

Some schools encourage covered-up styles. Others leave the choice wide open, limited only by inventiveness and budget. ("Can you imagine spending $200 for a prom dress?" lamented a distressed mother of a high-school senior. "It was the only really pretty, reasonably well-made dress we could find.")

Granted, the selection is less than varied if you look only at dresses showcased specifically for proms and graduations. Styling tends to be in the pretty-prissy genre (with some tilting towards slinky-sexy), but most are in the $40 to $70 price range.

But before you despair, try other departments and other stores besides the department-store variety. Here are some ideas:

Bridal departments for bridesmaids' dresses. (Remember that they often must be ordered in advance.)

Boutiques that feature regional dresses; for example, Mexican styles from The Phoenix, Welsh cottons from Laura Ashley, Indian dresses from Georgetown Cotton.

Sportswear departments, for separates or even a beach cover that might double as an easy-fitting dress.

And, surprise, lingerie departments, where designers like Mary McFadden, Geoffrey Beene, Bill Blass, Bill Tice can scale their price tags way down. (Because manufacture of these items is simpler, they can be made in factories with wage scales lower than those putting out designer clothes sold in expensive dress departments.)

And now for some of the choices being made around town:

Bethesda-Chevy Chase senior (and Woodward & Lothrop Teen Board member) Pam Walton said she had "no problem" about a graduation dress. "It will be short, but it will be covered up by a gown."

She admits, however, pondering the decision about a prom dress for some time. At the moment her choices are narrowed down to the traditional prim styles by Gunne Sax and "something not too old-fashioned, not even like a formal."

Paint Branch High School senior Beth Williams says that because last year's choice was a frilly and feminine princess style, this year she wants something "more sexy, without being provocative."

After checking some shopping centers her major complaint is that "They all look too much alike."

Walter Johnson senior Kathy Hahn knows that she doesn't want the "high-necked, tight-laced dresses a lot of stores are showing. And I know that I don't want black, and nothing really daring . . . I'm not built for it."

She is concerned about price, and the fact that she may wear the prom dress only once. Her favorite so far is from the bridal department at Garfinckel's in Spring Valley.

Holton Arms senior Karen Crowe has been thinking about something sleeveless, but not strapless, for graduation, in a light color with a flowery print. A cotton dress from Laura Ashley in a tiny book-lining print may be her choice.

Because cap and gown is the graduation costume for Woodrow Wilson High School seniors, Lisa Payne plans to get something "very simple" to wear beneath it.So far as the prom, she has in mind "something in a slippery crepe-like fabric, with a V-neck, maybe at the back, and a sash."

To fill the bill, she and several classmates have tapped another Wilson senior, Towanna Friend (who works without a pattern), to make the dress. "That way," says Lisa, "I'm sure to have something uniquely my own." CAPTION: Pictures 1 through 4, Pat Walton in two dresses from Woodies ($70,$72); Karen Crowe in a Laura Ashley ($60), and Kathy Hahn in her choice from Garfinckel's ($84). Photos by Craig Herndon -- The Washington Post