STAPLES, the business supply store, is now running a terrible ad. A father runs gleefully through the store picking back-to-school items off the shelves for his miserable children who stand glaring at him. In the background Perry Como sings a Christmas song, ''The Most Wonderful Time of the Year.'' Wonderful for the father, because he, a single parent perhaps, is now free of the constant presence of his darlings. But not a wonderful time for those who, having chosen the road less traveled by, now have to take care of the kids. This is the worst time of the year for teachers. August is the cruelest month. The third week in the month a letter comes from the superintendent, whose name you had almost forgotten. Dear staff member, it begins, Hope you're enjoying your summer. Yes, until this moment. The letter is the first tug back into the blurry bureaucracy of schools, the class schedules, faculty meetings, vapid terminology of the latest useless trend, the point-avoiding speeches by principals and tautological professors of education. You look at the letter for the date you have to be back, and throw the thing away. In other countries, and even in some American cities, school goes on all year. Which is better. Having the summer off to do whatever you want - to garden, travel, sit around and read books - is blissful. Worse than blissful, it is childlike. When it ends, the memory of your childhood ending, however that happened, is replayed. Dear staff member, Hope you're enjoying your summer. Please! Leave me alone! Can't you just send the checks and not bother me! I want to be 12 again! Is that so wrong? It's true that teachers work only nine months a year, but the schedule is inflexible. The school year cannot be changed, too many people are involved, too many buses and field trips and basketball practices and janitorial work orders and so on depend on it. The academic millstone grinds on, and nothing, not even education itself, can stop it. The work begins at 7 in the morning at a fevered pitch unknown among adults. Children are morning people, at least until 10th grade or so, and they squirm and jump, physically and mentally, demanding constant attention. So August dribbles to a golden ending, these few precious days. The summer dies with nothing done but relaxing or weeding or reading. Your non-teacher friends, if you have any, ridicule you, refusing to understand. Ahead is noise, the cacaphony of voices, the slamming of locker doors, the smell of sneakers and uneaten lunches and that awful mint-scented sawdust they give you to use when a lunch won't stay eaten. There are the incredibly odd phases of child psychology and bizarre daily dramas. (A girl in one of my classes wept uncontrollably. She had kissed a boy and was told by cruel friends she was pregnant.) There's the juvenile jockeying for status, arguing and fighting for position within the school's tiny society, some of this done by the students, most by the faculty. And there's the sense of getting relatively older. In regular jobs, co-workers share your chagrin at the passing years, but seventh- graders are always the same age. It takes two weeks at least until you make the transition from summer's blissful sloth to the work of teaching. Your mind is in your garden, not in your grade book. Kids float before your eyes as images to be denied. Reality comes at last in the form of a bomb scare, or a fight. But maybe, sometimes, something quite different. Maybe it comes when you're reading the beginning of ''Great Expectations'' to your class, where Pip meets the escaped prisoner in the graveyard, and you look up and, behold ... everyone's listening. Ah, well. Gardening, teaching, it's all the same. And besides, there'll be a frost soon.