Pam Shriver's delightful Christmas vacation came to an abrupt but instructive end yesterday.

The 15-year-old Baltimorean was scheduled to be back in the 10th grade at McDonogh High School yesterday, studying algebra and ancient history.

Instead, she took the day off to play 22-year-old Virginia Ruzici in the second round of the $100,000 Virginia Slims of Washington tennis tournament at Smith Center.

It was Ruzici who took young Shriver to school and taught her several lessons in close but firm 6-4, 7-5 victory.

"Losing is usually a better learning experience than winning," said Shriver, looking as though she were trying hard to believe it. "Perhaps I should have stayed back from the net a little more on my second serves and picked better shots to come in behind.

"Gee, she hits it with all the topspin like the European clay-court players do and that's going to take some getting used to."

Ruzici, who hits such a crushing forehand that she flies off her feet on every whack, broke Shiver's big seve in the very first game.

Shriver broke back for 2-2, but Ruzici cracked through immediately and served out the set.

Shriver, who stands 6 feet and is the talk of this tournament in light of her runner-up finish to Tracy Austin in the National Indoors 16-and-under, led the second set, 5-3, and was serving for the set.

"Boy, I sure could have taken that pretty easy" said Shriver sheepishly. "But I let it get away."

"Pam has never played a woman who moves around the court so well and has such a weapon as Ruzici's forehand," said Shriver's coach, Don Candy.

Shriver's shallow second serves allowed Ruzici to play around her weaker backhand and hit cannon-shot forehand service returns.

"Pam would follow her second ball to the net," said Candy, thinking of the horrible sight, "and she'd be looking down the barrel of a gun. We all have to have these painful new experiences."

Ruzici, a Romanian ground-stroker who jokes that she looks like a gypsy, upset sixth-seeded Wendy Turnbull in the first round and will meet the winner of Thursday's Betty Stove-Julie Anthony match in Friday's quarterfinals.

Shriver had considered using her Christmas vacation to go to California for the open prequaligying for the San Diego Avon Futures tournament. There she would have run into a prequalifying field of 152.

"That field was easier than the local qualifying field for this Slims tournament," said Shriver. "but I figured I'd stay close to home.

Yesterday, Shriver discovered that all her promising friends from the junior ranks had gotten blitzed before they got close to the Avon event, while she actually won a professional match (and then turned down the $1,500).

"That makes me feel better." said Shriver, cheering up. "Darn, I'm glad I stayed here."

Her first-round Slims victory over a stunned Pam Teeguariden gave Shriver 30 computer points and will ease her way into an Avon Futures event in Columbus, Ohio, later this month. To build that many valuable tour points, Shriver would have had to win the enormously crowded San Diego event by winning a dozen matches in a row.

"Maybe someday I'll be the No. 1 woman player in the world," said Shriver, a powerful serve-and-volley player. "I think I have what it takes. I know it seems like I'm very young to be thinking that way, but I agree with what Chris Evert says, that if you're ever going to be one of the best in the world, you'll probably show signs of it by the time you're 17 or 18 at the latest."

On this week's report card, Shriver received an A-plus.

Greer Stevens - nicknamed "Cat" because she 1) plays like one, 2) loves animals, and 3) is a fan of rock singer Cat Stevens - bested Fiorenza Mihai, 6-2, 7-6 (5-1) in the afternoon's other second-round mathc.

Stevens, only 20 and seventh-seeded, and Mihai, 22, must feel like old ladies in this tournament in which two high school students, Shriver and Regina Marsikova, who beat Billie Jean King, have created the biggest waves.