Last week, primary school teacher Katherine Yeo had hoped to book a trip to Osaka and Kyoto in Japan for her family during the upcoming school holidays.

But when she went to Dynasty Travel, Ms Yeo found out that flights alone would have cost about $2,500 a person.

In comparison, flights to Osaka during the off-peak season cost about $800 to $1,000.

"We kind of knew that we would not be able to get a good price for our trip," says Ms Yeo, who is in her 50s.

Eventually, she settled on an eight-day package to Taiwan instead, which cost just over $2,000 a person, including flights, accommodation and some meals.

A similar package during the off-peak season would cost about $1,500 a head.

Ms Yeo will be travelling in the middle of next month with her husband, 21-year-old son and 14-year-old daughter.

As one would expect, the school holidays are peak travel season for families with children and the travel industry knows it - marking up prices of flights, hotels and tours.

For those who have not booked their school-holiday trips, flights and tour packages are still available - albeit at higher prices, with limited seats and odd timings such as late-in-the-day arrival and early departure.

Ms Alicia Seah, director of public relations and communications for Dynasty Travel, estimates that airfares are at least 30 to 40 per cent more expensive than during the low or regular season.

For instance, flights to London or Sydney can cost up to $2,400 and $1,500 respectively - about $500 to $1,000 more than during the off-peak season.

However, not all travel agencies have the same mark-ups, with online travel agency Klook keeping its prices constant all year round. The company sells tickets for travel activities, including transport, food and attractions.

Mr Soong Chuan Sheng, general manager of Klook South-east Asia, says: "Travelling as a family during the school holidays doesn't need to be expensive. Klook's prices remain the same throughout the year."

To avoid not only paying a premium, but also the stress of booking late, travel agencies recommend booking vacations about six to nine months in advance.

Ms Jane Chang, head of marketing communications for Chan Brothers Travel, says: "Prudent travellers understand that if they wish to secure a preferred departure date at the best possible rate, the modus operandi is to fix their travel plans and schedule their bookings early."

Not everyone, however, can manage that. Ms Chang estimates that about 15 per cent of travellers book package tours less than a month from their departure date.

That proportion increases when it comes to free and easy travel.

Ms Robyn Lee, market growth lead from Skyscanner Singapore, says 44 per cent of flights in November and December last year were booked less than a month before the trip.

Ms Yeo, for instance, was unable to book the family vacation any earlier as she was waiting for her 14-year-old daughter to confirm rehearsal dates for her co-curricular activity - theatre.

Despite the higher prices, those who are able to travel only during the school holidays are willing to fork out the extra cash.

Ms Jerlynn Chiang, 27, paid a total of $322 for a flight to Bangkok on Jetstar Asia and a return ticket on Scoot. She will be travelling with her brother-in-law and his girlfriend in the middle of next month.

Ms Chiang, who is married, booked her flight late last month. She says the same flight would have cost $139 if she had booked it a week earlier and did not procrastinate.

Still, the primary school teacher did not mind paying extra as she just wanted to leave Singapore for a holiday.

Her husband, an analyst, will not be joining her on the trip as he does not have any more annual leave.

As for Ms Yeo, paying a premium is worth it for the time spent with her family.

She says: "The children are getting older and soon it won't be so convenient for them to go on holiday with us.

"My son is going to university next year, so now is the right time for us to travel together before everyone starts going off on his own."