BOSTON (AP) _ Lawyers beware. If you don't do your homework, U.S. District Judge Walter Jay Skinner could send you back to school.

Exasperated by what he considered sloppy legal work in a case before him, Skinner has ordered two lawyers to attend an all-day seminar on federal practice sponsored by the Masssachusetts Council for Legal Education.

He warned that ignoring his order ''will lead to more serious sanctions.''

''Perhaps some appreciation of precise pleading will rub off on them, if they have not already gotten the point,'' Skinner said.

A third lawyer in the case said he, too, is responsible, and has written the judge a letter volunteering to attend the Sept. 26 seminar at Boston College Law School.

Legal experts said the judge's action was unusual. Some applauded it, saying it should serve as a warning to lawyers who go to court unprepared.

''It's cute,'' said Harvard law professor Abram Chayes. ''I think the attorneys will be embarrassed, and they ought to be embarrassed.''

In his July 9 order, Skinner accused lawyers Frank. A. Smith and Jerry B. Plumb of carelessness in drafting pleadings in a civil suit against the city of Brockton, about 20 miles south of Boston, and its police chief.

In the suit, former Patrolman William M. Stevens Jr., is seeking damages from Brockton and Police Chief Robert. D. Gillis, alleging harassment led to the loss of his job.

Skinner found fault with two motions presented on Stevens' behalf by Plumb and with one filed by Smith for the city.

A motion against Brockton claiming slander and intentional infliction of emotional distress had no legal basis and ''a moment's research would have made clear that the city was immune from liability,'' the judge found.

A motion against the city and Gillis claiming wrongful termination of employment was too broad, and ''it should have been abundantly clear that only the city was liable for such negligence,'' said Skinner.

''Defendants were equally reckless,'' he said.

Their move to have Stevens' claim against the city and the police chief for slander and infliction of emotional distress thrown out, and to dismiss the claim against the city for wrongful termination, ''was clearly groundless,'' the judge found.

Smith, 39, who said he has been practicing for 10 years, would not say if he will comply with Skinner's order. ''There is still time to appeal,'' he said.

A call to Plumb's office was taken by James. A. Frieden, Plumb's partner. He said he was essentially at fault, and ''Jerry had nothing to do with the way the complaint was written.

''I'll be more careful in the future, and I think Jerry will be, too,'' said Frieden, adding he sent a letter to Skinner explaining that ''Mr. Plumb was not the guy who did it. It was me.''

Frieden, 39, who said he also has been in practice for 10 years, volunteered to join Plumb at the seminar.

While some legal experts considered Skinner's action irregular, they said it was valid and probably more effective than levying fines, the usual course of action.

''This is much more pointed and is likely to have a more long-term effect,'' said Harvard law professor Andrew Kaufman. ''Other lawyers will hear about this and won't want to get a slap in the face.''

Those less enthusiastic include Mike Moen, former president of the Massachusetts Academy of Trial Attorneys.

''I think that was an order that rises out of frustration that judges feel when they see unnecessary pleadings,'' said Moen. ''Unfortunately, these two lawyers were the ones standing there at the point frustration reaches a boiling point.

''I think that holding out these two lawyers out to embarrassment is unfortunate, because they are not the only ones guilty of such carelessness.' '