With each train arriving at the Arlington T station in downtown Boston, alternating waves of high school students clad in T-shirts and jeans and Harvard first-years in black tie and ballgown finery flooded into the lavish mezzanine of the Park Plaza Hotel Saturday night.

After some confusion, they understood what was going on.

The Harvard First-Year Formal and the Harvard Model Congress (HMC) dance were taking place in opposite corners of the Park Plaza at the same time.

HMC is an annual government simulation run by Harvard undergraduates. This year's HMC drew about 1,500 high school students from across the country.

But by Saturday night, most of the formal committee work was done, leaving the high-school students free to dance the night away. With their transformation came another, as Harvard students switched from committee leaders to party chaperones.

Model Congress security agents patrolled key locations throughout the hotel, complete with radios, ear pieces and serious demeanors. The agents prevented anyone without proper HMC identification from entering their dance and kept high-school students from checking out the Harvard first-years.

Around 10:30 p.m., reports of noise on floors six and eight were relayed to members of security, who rushed off to try to discover the source of the disturbance.

On their way, they discovered a breach of security--it was possible to enter the First-Year Formal from the HMC dance through a banquet kitchen. "We have to secure this area," one of them said, and the others agreed.

Such security measures not only made the simulation more realistic, but also kept misbehavior at a minimum.

"Given the age group, behavior has been exceptional," said a supervisor in the Park Plaza's food and beverage department, who declined to state his name. "I don't see why we wouldn't have them next year."

Participating students said that the carefully orchestrated weekend mixed learning with fun.

"We had a great time," said Michal Benzaquen, a ninth-grade student at the Hebrew Academy of the Five Towns and Rockaway in Lawrence, N.Y.

"I think it's a lot of fun," said Gabriella L. Pollak '00, chair of the Senate Committee on Environmental and Public Works. "They realize how long it takes to get a bill passed."

Tali Koss, an 11th grader at the Hebrew Academy, concurred. "We learned how nothing gets done."

"It's been fantastic for them," said Carol Smyth, faculty adviser for delegates from the West Tresper Clarke High School in Westbury, N.Y.