The 10 girls, all 17 or 18, booked a holiday at the four-star resort of Laganas on the Greek island of Zante.
But after making regular payments to the Glasgow Fort branch of First Choice they were told their carefully planned holiday had been wrecked because of "human error".
They say they were then offered two-star accommodation 10 miles away from the hotel they originally booked and a small discount from the full holiday price.
But they say the travel agent is now charging them extra to take 20kg of luggage with them – which they say was already agreed in the original booking. That means the holiday is now costing the same as the original.
The friends organised the trip last October with the help of their parents.
But when one mother went to pay the remaining balance last month the shop manager told them one of her staff had failed to confirm the booking.
She then offered the group the alternate accommodation or a refund.
But Tracy Leonard, whose daughter Paula, 17, is booked on the trip, said they believed it was too late to start looking for alternate resorts and felt pressured into accepting the two-star hotel.
She said: "We were offered the chance to cancel the holiday, but the girls wouldn't get their money back for five days. This meant they didn't have the funds to book another holiday elsewhere.
"They were so desperate to get away for a holiday, the girls felt they had no choice but to accept it.
"When we first booked the holiday last October, all the girls were only 16 and 17, so a few of the parents, including me, were apprehensive about letting them go.
"But when we saw the four-star hotel, and where it was, it looked really good and safe, so we agreed. Now I am really worried about Paula going away.
"The original hotel was right in the centre of town. This other hotel is miles away."
The girls have all just left St Ambrose High, Coatbridge, and planned the holiday to celebrate leaving.
Paula, Caitlin Tierney, Courtney McKenzie, Shannon O'Connor, Abbie Blyth, Emma Claire Cooper, Samantha Hamill, Erin Christie, Rachael Shannon and Amie Wallace are due to fly out to Greece in July.
Paula, who will study business at Glasgow Caledonian University after the summer, said: "First Choice offered us different accommodation and basically said, 'Take it or leave it".
"We really had no choice as we are desperate to go away together. I still don't understand how this could have happened when we were all going into the shop and making payments.
"We trusted this company and all it has done is let us down."
A spokeswoman for the travel agent said: "First Choice would like to apologise to the party for the issues they experienced.
"We understand how disappointing it is when holidays are changed close to departure.
"As a gesture of goodwill we would like to offer them a complimentary excursion. They will be able to arrange this with our holiday advisers in the resort.
"Our retail shops make thousands of holiday bookings a year and issues of this kind are rare."