The headmasters at seven of the Washington area's most prestigious private schools have written a letter to the parents of all students warning them that students are regularly throwing large, unsupervised parties where "excessive drinking and sexual license are common."

In what the headmasters called a rare joint effort, the letters, which were mailed Thursday, asked parents to step up supervision of their children to prevent them from attending or throwing weekend parties that are open to almost anyone and where alcohol is easily available.

The letter was written jointly "to give it more impact," said Malcolm Coates, headmaster at Landon School in Bethesda. "The fact that seven schools decided it was enough of a problem to address it is significant."

Individual schools have confronted the issue before. At the beginning of the school year, for example, Georgetown Preparatory School in Rockville held a conference with parents to discuss the problem of unsupervised parties and similar activities.

One solution, a parent said, was the creation of parent groups to open up communication among parents and lines of information about where their children are, what they are doing and with whom.

The headmasters said they believe that on most weekends there is at least one large party, sometimes with several hundred students in attendance.

The letter said, "It would be hard to devise a better recipe for disaster than a social scene that includes the anonymity provided by an 'open party,' no adult supervision, considerable amounts of alcohol, and teenage hormones which encourage sexual or violent behavior."

The two-page letter was signed by the headmasters from Georgetown Preparatory, Landon, Gonzaga College High, National Cathedral, Holton-Arms, St. Albans and Sidwell Friends schools.

"Over the past few years, we've seen an increase in the sort of large, open parties which are not held at school, and involved students from a number of schools," said St. Albans Headmaster Mark H. Mullin. "Most of us have a pretty good feel for the generalities of what goes on over the weekend."

Each of the six headmasters interviewed yesterday said no one incident had sparked the letter. The issue of unsupervised parties was raised during a January breakfast meeting among the headmasters, one of them said. After a discussion of their mutual concerns, they decided to write the letter.

"I'm very pleased with the letter," said Judith Volcko, vice president of the Mothers Club at St. Albans, in Northwest Washington. "I send my kids to private schools because I want the school to back up my husband and my feeling on discipline. I want the same message given at school that is given at home."

Charles P. Lord, headmaster at Holton-Arms, in Bethesda, said, "A number of parents and kids have expressed dismay over some of the situations at weekend parties."

These have included incidents of vandalism, fights and property damage within homes.

"This year, at my house, it got pretty bad," said a senior at Maret School in Northwest Washington. Although Maret was not represented in the letter, it is part of a circuit of private schools where students interact socially. "It got out of control," the senior said. "I intended for it to be an open party, but I didn't intend for people I didn't know to come."

"There's always an empty house," said Patrick Welsh, an English teacher at T.C. Williams High School in Alexandria, who has studied excessive drinking among teens.

"It's a middle-class phenomenon," he said of the elaborate parties that can involve hundreds of guests and may be organized in as little as a day when word gets out that parents are out of town.

Caleb Shreve, a senior at Sidwell Friends in Northwest Washington, said he threw a party this year that saw as many as 500 people stream through his back yard, where he had three kegs of beer.

"I haven't been to a party this year where there were fists thrown," he said. "Last year and the year before, there were quite a few fights." He and the senior at Maret said the parties have settled down and gotten smaller recently.

"That's wonderful news if it's true," Lord said. " . . . If the letter helps to settle it down further, that's even better. We're concerned about the potential for tragedy."

Special correspondent Andrew C. Lottmann contributed to this report.