An informal survey of about a hundred teenagers who attended high-school proms last year shows that the traditional end-of-the-school-year formal affair is still the event of the school year. What were considered essentials ten, fifteen, twenty years ago or more are still considered essentials today. In the case of prom-related activities, the present mirrors the past.

For boys: Rent a tux. Get hair cut 10 days early so it grows out just right and just in time for the big night. Dinner reservations. Plan the beach trip for the next day. Wash the car, your hair and your neck. Wash the tux shirt to prevent starch burns. Brush up on manners. Watch Mom's eyes mist over as you walk out the door.

For girls: Buy a gown. Buy some shoes to match the gown. Buy a new bathing suit for the trip to the beach. Don't forget the boutonniere. Hope he's remembered to make the dinner reservations. Keep the new hairdo in place from midday until the evening. Watch Dad's eyes mist over as you walk out the door.

Sure it sounds familiar. And May is still the month of prom-provoked whispers, thoughts and glances. That nondescript person in the back of the geometry class has mysteriously begun to acquire personality and attractiveness. The person is no longer considered dull; now the person is considered shy, demure, perhaps the strong, silent type. The person has transformed into a prom prospect.

The date is made. Immediately, preparations begin. The traditional events of prom weekend cost a little more today, and the prom is likely to be held in a rented ballroom rather than the decorated and disguised gyms of the past. The average teenager will drop $80 to $100 over prom weekend, so it's important to do things right and have a good time.

Besides, it's Prom Time - something everyone's supposed to remember fondly for a long time. Here are some suggestions, gleaned from last year's promsters, on what to look for in prom essentials:

THE TUX - Most schools set up an arrangement with a rental shop near the school to come and take measurements. Hard as it may be to believe, tuxes don't have to be uncomfortable. Go ahead and ask how heavily starched the shirts are and what kind of material the pants are made of. No white socks, please. And wear plain black shoes - they can be rented too ( $6). The price range for tuxes from the crowd interviewed was between $32 and $75. The $75 outfit included top hat and cane. Most shops carry colors other than the traditional black, gray and white.

THE GOWN - The most expensive gown worn by a participant in the poll cost $75, the least expensive $25. The difference was that the $25 gown was homemade, a good way for the handy types to save some money. One girl spent no money on her gown - she simply wore the one she had worn the year before.

Gowns, of course, can't be worn with daily footwear so, invariably, a cost factor for shoes must be considered. Footwear ranged from $12 to $38. The same girl who wore last year's dress also wore last year's shoes, noting proudly, "Nobody knew the difference; besides, I liked the outfit."

Several girls advised buying something that wouldn't be too difficult to dance in, since most prom bands are of the rock'n'roll variety.

THE DINNER - All-important in creating the right effect. The biggest complaint about restaurants was not the price - they indicated a willingness to spend ample amounts of money ( $15 to $38 for two) - but the tendency for certain restaurants to treat them "like kids." Nothing puts a damper on a night out like poor service in a restaurant - especially when decked out in a tux or gown. Teenagers might think about telling the restaurant ahead of time that they're prombound. If the party on the other end emits a groan, make reservations elsewhere.

Restaurants with a view, restaurants in Georgetown and restaurants with foreign names seem to be favorites for the prom crowd. Following are some of the places that got favorable comments from last year's group:

ALFIO'S LA BELLA VISTA - (1011 Arlington Boulevard. 525-9195. A great view of Washington. Nice people. $25 for two.

NATHANS II - 1211 Connecticut Avenue NW. 659-1211. Received the highest compliments of the survey by a party of four that dined there. They liked the variety of Italian food, the atmosphere, the huge chandelier over the bar. Bill averaged about $15 per person.

ALEXANDER'S III PENTHOUSE - 1500 Wilson Boulevard, Arlington. 527-0100. Nice surroundings, lovely view, good service and good meal. $22 for two.

TOM SARRIS ORLEANS HOUSE - 1213 Wilson Boulevard, Arlington. 524-2929. "It was a good dinner. Especially the strawberry daiquiris and the wine and the salad bar." $30 for two.

PORT O'GEORGETOWN - 1054 31st Street NW. 338-6600. "Decent!" (Teens will, of course, recognize this as a highly favorable term.) $23 for two.

THE NEXT DAY - It generally begins with breakfast at someone's house, a nice touch. One girl had a group of 30 over at a cost of about $50. They all caravaned to the favorite post-prom spot - Ocean City - for the day. It's a long trip for weary drivers. Teens warn that it's a good idea to swap turns driving. Picnics at Great Falls, lunch and matinees in Washington, a light-hearted trip to the National Zoo and a day-long canoe trip also received high marks as day-after activities.

LITTLE EXTRAS - The things that make proms memorable. Pictures. Flowers. A charm inscribed with the date. Saving the program. Last year's most original extra: "A small bottle of Chanel No. 5 for the lady." CAPTION: Picture, no caption, By Craig Herndon