"I'm competing against a great number of students across China," says the 18-year-old. "In top Beijing schools like the High School Affiliated to Renmin University of China, one third of the whole [form] are preparing for the SAT.

"Some schools even have a couple of designated SAT classes, whose students spend all day in the library preparing for the test on their own."

The US is perhaps the most popular destination for mainlanders hoping to study abroad. This year, US universities admitted 93,789 first-year students from China, which overtook India and Canada to become the country with the largest number of undergraduate students in the US.

But the SAT, offered six times each year in 175 countries and regions outside the US, including Hong Kong, is not generally available to students on the mainland.

Only those attending international schools may take the test in their home city. Which is why tens of thousands of students like Wang travel across the border several times a year to sit for the SAT in Hong Kong, the nearest exam centre.

Another wave arrives today, ahead of the tomorrow's SAT sitting, although numbers are lower than in October, which is the peak period for college admission tests.

Since AsiaWorld-Expo was chosen as an SAT exam centre in October 2011, the increasing numbers of mainland candidates have proved to be a windfall for a range of business beyond the exhibition centre near Chek Lap Kok airport.

(Before the centre was pressed into service, SAT tests were held in school halls across the city, most with capacity for up to 200 people.)

AsiaWorld-Expo chief executive Allen Ha says more than 10,000 candidates sat for the SAT at the centre last month, over 90 per cent of them from the mainland. It took more than 600 people to set up the halls and monitor the exam held on October 5, mostly temporary staff recruited for the event.

Catering services at the exhibition centre as well as hotels and shops in nearby Tung Chung also benefited from the influx of mainland candidates, many of whom came to Hong Kong with their families.

"Quite a number [of families] went shopping after the test to reward the student. They are high-end customers who can afford to send their kids abroad," Ha says.

Education agencies have also seized the opportunity to hawk their services, as Wang found when she emerged from her test last month; she found hordes of salesmen outside waving promotional pamphlets for various courses and US colleges.

"Many booklets are printed in simplified Chinese, and most of the sales staff are talking in Putonghua," she says.

Driven by soaring demand, rates for a deluxe room at Skycity Marriott, a mere three-minute walk from the centre, almost doubled from HK$1,600 to HK$3,000 during the exam period.

"We enjoyed full occupancy during exam weekend last month," says the hotel's marketing director Elizabeth Wan.

"It was the time for family tours. Entire families, including grandparents, came to support the students."

The nearby Regal Airport Hotel also recorded a 15 per cent increase in occupancy on exam dates. The Regal Hotel's area general manager, John Girard, says SAT candidates and their families account for 30 per cent of guests on exam weekends.

The enormous demand caught Tao Tao and his parents by surprise. The family from Beijing tried to book rooms online at Skycity Marriott in mid-September, but could only find two deluxe suites costing more than HK$5,000.

Difficulties faced by families such as Tao's have boosted a SAT tourism business on the mainland. Education agencies like New Oriental, known for its SAT training courses, now offer exam travel packages.

A website touted the agency's "SAT Full Score Tour", which included round-trip air tickets, a standard room at a four-star hotel, shuttle bus services to the exam centre, "and an English teacher specialising in SAT to answer all your questions about the test until the last minute". Prices ranged from HK$5,000 to HK$7,000, depending on the length of stay.

"We have a three-day standard tour, a four-day 'happy tour', and four-day mock test tour," says a teacher from New Oriental, who brought 30 students to Hong Kong for the last sitting. "About 300 New Oriental SAT tour groups departed from Beijing in October."

The school also provides plenty of choices for students from other major cities on the mainland, he adds.

When Eric Zhang flew to Hong Kong for his SAT in October, he found his waiting lounge at Beijing International Airport was filled with New Oriental candidates.

The student from the High School Attached to Capital Normal University says only two out of the 30 students in his "international" class chose to travel independently to take their SAT test.

"I felt so lucky to secure the last seat. Someone dropped out from the October tour when I got to the booking centre, or I would have to travel by myself."

Zhang didn't mind making the trip to sit for his SAT as he found it "as fun as a school trip", and spent the last day of his tour seeing the sights in Hong Kong with his classmates. But he says it is unfair that only students from international schools can sit for the exam on the mainland.

China's Ministry of Education authorised 50 international schools across the mainland as test centres for international school students.

A spokesman for the College Board, the American institute that designs and hosts the SAT exams, says its policies and procedures regarding testing "are to maintain compliance with these guidelines".

But for students at public high schools, exam centres outside the mainland are a required first stop before they can head to the West.

Wang Yaohui, director-general of the Centre for China and Globalisation, a Beijing think-tank, says limiting access to SAT tests is a way to prevent more top students from going abroad for tertiary education.

"At the moment, a family has to invest a considerable amount of money, time and effort to send a child to sit the SAT outside China. Once the barrier on SAT is lifted in major cities, many more top students will fly to the States, voting against Chinese tertiary education with their feet," he says.

That's not to say job prospects are better for mainland graduates with degrees from abroad. The education ministry has reported that 270,000 graduates returned to China last year, double the number in 2011, making the past job-hunting season the "toughest in history".

But Wang says US colleges remain attractive because top mainland universities still lag far behind in terms of diversity and globalisation. "Foreign students and scholars make up only 1.4 per cent of the total population in top universities in Beijing. But the top 20 American universities have around 20 [per cent foreign scholars]," Wang says.

Students such as Zhang also see other benefits. "The admission process for American universities is much better than that on the mainland, where one single exam determines our destiny," Zhang says.