Susan Wenner heard a knock at her classroom door as she was explaining the federal court system to T.C. Williams High School students in Alexandria.

As she opened the door, a tall stranger asked whether she was Mrs. Wenner. When she nodded, he pulled an 18-inch stick from behind his back and struck her repeatedly on the head, police said yesterday.

"He started asking me questions about papers and students," Wenner said.

The 34-year-old teacher suffered a 2 1/2-inch gash on the forehead in the Tuesday morning incident. She was treated at Alexandria Hospital and released.

The attacker, thought to be 20 to 25 years old, fled the building before Wenner's shocked 11th grade students could move, police said. Alexandria police spokeswoman Lucy Crockett would say only that "we are investigating the leads we have."

School officials, who confirmed the incident yesterday, said the suburban high school school of 2,468 students was shaken by the attack.

"Our first reaction is one of disbelief," said English teacher Cynthia K. Pennell. "It's just so outrageous that it could happen here."

Principal John Porter and other school officials said they believed it was the first time a teacher has been assaulted on school property in Alexandria.

Wenner said yesterday that she felt shaky because "it was premeditated. He asked for me by name."

Last year, Wenner moved to Alexandria from Houston where she was also a teacher.

Porter said the halls, which are regularly monitored by adults, will now be more aggressively patrolled. All those who look suspicious or are not students will immediately be directed to the main office, he said.