BAO ZIER is talking to an adult education class of 10 students at Falls Church High School. Softspoken, patient and extremely tidy, she seems to be an ideal instructor in her subject, Thai cooking. And so she is. Then, in a moment, the room is gripped by tension. She has produced a powerful weapon and is about to use it. Slowly she spoons some home-ground chili pepper into a saucepan. And some more! And some more!

Thai cuisine is among the hottest in the Orient and Bao Zier is loyal to its precepts. She's not going to compromise on flavor, and her students love her for it. On the other hand, she is no snob. "You need special ingredients to do Thai cooking," she explained, "but they are easy to buy in Washington and once you have them the cooking is not difficult or time consuming. You don't need special equipment and we eat with fork and spoon instead of chopsticks."

Don Fisher, one of Zier's most enthusiastic students, explained his fascination with Thai cooking this way: "To me it is a medley of Indian and Chinese. Some of the dishes are dead-ringers for Chinese, but I've found a sameness in Szechwan seasoning that I don't find in Thai dishes. I like Thai curries better than the Indian curries I've had in this country. The Thais use seasoning pastes instead of powdered spices and (they) are less raw-tasting to me."

Thailand is a tropical country in which fruit and vegetables are abundant as are fish and seafood. No Thai considers a day's eating complete unless it has included a blow of rice, and at most meals a bowl of fresh fruit is nearby. Frequently the midday meal is the largest of the day and meals for company are likely to feature elaborate garniture and decoration rather than special dishes. Fish sauce takes the place of soy sauce in many dishes reminiscent of the Chinese. Soups and sauces tend to be clear, not thickened with cornstarch as they would be in China. Coconut milk is used extensively for sates which are very popular, but unlike the Indonesians, Thai cooks find peanut oil too heavy for their cooking.

But if Thai cooking is a polyglot cuisine, it does have a symbol: the chili pepper. Red or green; several inches long or the size of a thumbnail; whole, split or crushed, there is no escaping them. Nor should you try. Practice moderation, perhaps, but abstinence would be unfair. Don't feel cowardly for avoiding or eliminating pepper seeds, however. They can blow off the top of your head.

Chilies were a subject of discussion at the Zier home on a recent Sunday afternoon. The Zier's two daughters, 10-year-old Pookie and 8-year-old Vena, have been gradually introduced to spicy fare. Their American father, Bill Zier, is thoroughly indoctrinated from years of engineering work in the Orient. "When I first visited Bangkok in 1965," he recalled, "I tried a bowl of fried rice with lots of little red and green bits in it. I was told it was sweet, but it was hot; the hottest thing I'd ever had. I like it now, but it took awhile and then I discovered the food in Bangkok is mild compared to what they eat upcountry."

The meal began with chicken wings stuffed with a mixture of seafood, vegetables and cellophane noodles. There were chilies, of course, chopped and added to a sweet-and-sour dip sauce. Next came pork sate with a peanut sauce Bao Zier makes from scratch by raosting peanuts in a wok and then grinding them. A side dish of cucumbers in a sugar-vinegar-water mixture appeared, looking cooling and inviting until one came upon a chunk of red chili pepper.

A superb soup of zucchini and pork balls was followed by the main courses. There were three, more than normal for such a meal: chicken fried with ginger and red and green peppers, a chicken curry and beef with basil.

Beer and rice were available in quantity. At the end came some cooling fruit in syrup. It was a pretty meal, one full of taste as well as tang.

"If you roast your chilies before you grind them, you can smell the difference in the dish," Bao Zier had explained to her students, and it proved true. "If you burn them the smell is awful," volunteered one of the students. "Yes," answered the teacher. "But you have to roast them until they turn black. I open all the windows when I do it here or it hurts my eyes.In Thailand many of the cooking facilities are under a roof out-of-doors, so it's not such a problem."

Bao Zier had brought her groceries with her to the class in a shopping cart. They included fresh coriander, of which she used the root (or stem) in two preparations, and a large "green papaya." It is the size of a small melon and unlike the fruit available from Florida and the tropics, the green papaya is not sweet. She chopped and cut the pulp into shreds for use in a salad.

In addition to a knife and cleaver, her chief tool was a worn mortar and pestle, used to grind ingredients for sauces and marinades. She uses a blender at times, but seems to prefer the old method. Chicken to be broiled was cut further to expose more the flesh in wing, leg and breast. That meant more surface was exposed to a coconut milk marinade and cooking time was shortened.

The students listened, watched and participated in various preparations. Several of them had lived in the Orient and all agreed they had developed a new appreciation of spicy food and indeed were making the recipes at home. Bao Zier kindly encouraged a student who hadn't done something well the week before to try again.

She is scheduled to give the following classes at Falls Church High School: Thai Cooking I (six classes for a total fee of $25.50 on Monday evenings at 7 p.m. beginning the week of Feb. 4), Thai Cooking II (six classes for a total fee of $28.50 on Monday evenings at 7 p.m. beginning April 7) and a course in Thai appetizers (four classes for $19.50 at 7 p.m. Wednesdays beginning Feb. 6). For further information, call the Annandale Adult Center at 256-8448. KAENG CHUD (ZUCCHINI SOUP) (6 to 8 servings) 1/2 pound ground pork 1/2 teaspoon salt 1/2 teaspoon garlic powder 1/2 teaspoon monosodium glutamate (optional) 1/2 teaspoon black pepper 1 1/2 pounds fresh zucchini, cut into small chunks. 5 cups of water with 1 chicken bouillon cubs, or 5 cups chicken stock 2 teaspoon preserved cabbage or salted Chinese cabbage 2 to 3 tablespoon fish sauce or 1 teaspoon salt

Mix pork, salt, garlic powder, 1/4 teaspoon optional MSG and 1/4 teaspoon pepper and set aside.

Boil water or stock in medium size pot. Add preserved cabbage, fish sauce, remaining black pepper, chicken boullion and remaining MSG. Form 1/2 teaspoon of meat into a ball. Repeat until all meat is used. Drop balls into boiling liquid. Cook until meat rises to surface. Add zucchini and cover. Cook 3 minutes on high heat, serve hot. KAI YANG (CHARCOAL GRILLED CHICKEN) (4 or 5 servings) 1 whole chicken, cut up, washed and patted dry 5 large cloves garlic, crushed 20 peppercorns or 1 teaspoon black pepper 4 large stems of fresh coriander, chopped 4 to 5 tablespoons fish sauce 1 teaspoon monosodium glutamate (optional) 1 can (12 ounces) coconut milk or coconut cream)

In a blender grind garlic, peppercorns, coriander, fish sauce and optional MSG until smooth. Measure 3/4 cup of coconut milk and pour it into large bowl. Add garlic mixture and stir. Put chicken pieces in the bowl. Let chicken marinate in this mixture for 30 minutes or more.

Grill chicken about 15 minutes on each side in the broiler about 4 inches from heat. Serve as a main dish or as a side dish with chili sauce as a dip. CHILI SAUCE 1/2 cup vinegar 5 tablespoons sugar 1 teaspoon salt 3 to 5 chili peppers 1 large stem of fresh coriander, chopped 3 large cloves garlic, crushed

Using blender, grind all ingredients until solids are finely chopped. Pour the mixture into a small pot. Cook over medium heat for 7 to 10 minutes.Serve with grilled chicken. PAPAYA SALAD (4 or 5 servings) 2 cups (1/2 pound) shreded papaya, carrots, or cabbage 3 cherry tomatoes, cut into quarters or 1 small tomato, cut into small pieces 1 large clove garlic 2 small chili peppers, or 1/4 to 1/2 teaspoon chili powder 1/4 teaspoon monosodium glutamate (optional) 2 to 3 teaspoons sugar 2 tablespoons fish sauce 2 tablespoons fresh lime juice 1 tablespoon ground dried shrimp (optional) Fresh lettuce for garniture

Using mortar or unbreakable bowl, pound garlic and chili peppers until finely ground. Add shreded papaya and tomatoes and then pound several more times to release juices. Add optional MSG, sugar, fish sauce, lime juice and ground dried shrimp. Mix together well.

Serve with grilled chicken, fresh vegetables and rice cooked in coconut milk or sticky rice.