MANY PARENTS who have a tough time getting through school would find that if they could only master Basic Deportment, the rest would be a breeze.

Disapproving teachers, the nightly burden of homework, shame over less-than-perfect grades, the embarassment of reciting poorly in front of others -- all these problems can be lessened, if not erased, if only parents could learn to behave like little grown-ups.

The children, of course, would still have all these difficulties. That is in the nature of things, and the memory of difficult school years gives a person a rich feeling of satisfaction all through life at having grown beyond their reach. But parents can escape it and benefit their children at the same time.

Take the matter of reciting, which is what we call discussing new subject matter until one's ignorance is apparent to all. Most parents, when their children report having learned something at school, feel as if they have been called upon in class. Instead of listening to the children's newly acquired knowledge, as politeness demands, they take the mention of the topic as a direction to tell everything they know about the subject, thus not only squelching the child's pride, but eventually often getting themselves into that awful situation when the child reports that the teacher's version differs convincingly from theirs. Or homework. If the parent takes over responsibility for homework being done, which is not the same thing as supplying modest help on request, the parent has relieved the child of a difficult burden and taken it upon himself.

It is another common rudeness for parents to criticize grades automatically, even if they are good ones. A child who brings home a 96 rarely is congratulated; he or she is asked, "What were the four points off for?" or told, "Next time, see if you can make 100."

Miss Manners generally admires family loyalty, but is amazed at the number of parents who come charging into schools to do their children's battles without having a second source for the facts of the situation. And their first one is always what is known in Miss Manners' trade as "a usually unreliable source." All parents know that the reason educators are paid so little, in spite of all our believing that the job of educating the young is the most important task there is, is that teachers are rewarded enough by being allowed to spend their nights and weekends reading their adorable papers. Nevertheless, she thinks it a matter of elementary manners to treat a teacher with some respect for the profession, which involves not circumventing, or helping the child to circumvent, the rules and requirements of the classroom. Teaching is cursed with being one of the jobs like movie-making or being president, that every layman is convinced he could do better than the professionals.

And perhaps if parents promise to behave themselves the teachers will let them off easily, and not send home notes requiring them to produce bunny costumes, $5, three empty milk containers or themselves by 10:45 a.m. tomorrow. c MISS MANNERS RESPONDS

Q. My husband and I have a serious situation facing us. My son, 29, is being married and we, his parents, were just informed that it is our duty to give a rehearsal dinner for the wedding party. There are 16 in the wedding party, plus, we're also told, we must invite the various spouses or mates of the individuals participating.

My husband and I are living on a fixed income, and in today's inflation, we cannot come up with any solution of how we can get around this issue of feeding 32 people because of "tradition."

Our son tells us we're not up on wedding etiquette. Miss Manners, will you please help us in this plight?

A. Tradition, indeed! Miss Manners has no patience with people who invoke etiquette for the purpose of bilking others, epecially if they happen not to know what they are talking about.

The tradition, you may inform your smart-alecky son, was always that the bride's parents gave a bridesmaids'-and-ushers' dinner or, more likely, a tea after an afternoon rehearsal. Only in recent decades has it become usual for the bridegroom's parents to give a rehearsal dinner for the wedding party, their spouses and the clergyman and his wife, in order to free the bride's parents from this obligation at a time when they are already harrassed beyond human endurance, and to create a role for a family that was traditionally nearly invisible at the wedding, namely the relatives of the bridegroom.

In any case, prolonged carousing the night before a wedding is a terrible idea. Miss Manners suggests you give a tea party after the rehearsal, tea and cucumber sandwiches being both festive and cheap. Or you could do the traditional thing and let the bride's family worry about entertaining all those people.

Q. Just recently, I received a note from my cousin which began, "Bill and I would like to thank you for the graduation gift you gave to Tom." I was beginning to wonder whether Tom, a graduate of a Roman Catholic high school in a college town in Minnesota, had not been taught how to write. I myself received a reasonably good education in the Minnesota public schools.

My fears were allayed, however, when I received a cancelled check endorsed "Tom Smith" (not his real name). The check had been made out to Thomas D. Smith, as I understood his full name to be from the graduation announcement. The check was cashed by the local liquor store.

Perhaps Tom, only 18 and unaccustomed, we hope, to strong drink, spent the entire sum of $25 to sample some of the many varieties available, and as a result was temporarily unable to lift a pen.

I wonder if Miss Manners has any comment on all this. Am I being terribly old-fashioned to think it would have been more proper for the young man to express gratitude himself, whether or not he felt any?

A. As Miss Manners understands it, the phrase "being terribly old-fashioned" is the apologetic way in which people admit to a timid, hopeless desire to be treated with common decency by the young. It is interesting to observe how your cousins are attempting to live a double standard, acknowledging the propriety of your being thanked, but not imposing the necessity for doing so on their son. All this craveness is, in Miss Manners' opinion, why there is crisis of manners in the world today.

Of course, the young man should have thanked you.

(Actually, a hangover is an excellent time for a nice, quiet activity such as writing thank-you notes, if one can stand the sound of the pen's scratching on the paper.) Generosity and gratitude should always travel together, and since the gratitude is absent, Miss Manners suggests you squelch the generosity.

Q. I work in the intensive care unit of a hospital. Though many patients survive, many are injured or sick beyond medical help, and die. When family and friends call asking the condition of a recently deceased patient, what is the proper verb to us?

My etiquette-conscious mother ingrained in me never to say "passed away." Now I cannot remember more than grammatical reasons for avoidance of the term, but the note of disgust in her whenever she heard the phrase remains with me.

"Died" is the word I was taught to use, but I sometimes wonder if a gentler euphemism would not be kinder, under the circumstances. The medical term is "expired." However, that word seems too esoteric. Please advise me. t

A. There are euphemisms and euphemisms. Miss Manners agrees with your mother about "passed away," which suggests to her the train she just missed as it pulls out of the station, but does not eschew delicate phrases entirely. Nobody wants a vivid description, complete with active verbs, of what a person leaving a dinner table is about to do. "Expired" is both impersonal and clear.

Euphemisms are worst when they are so elaborate as to call attention to themselves. If you told a relative that a patient was "no longer with us," he might conclude that the victim had checked out from the hospital. "We lost him" is even worse, suggesting as it does, that the next step is to find him.