The makers should have come up with a better excuse keeping Preeti away form the trip. This makes no sense.
But, keeping Preeti away is mainly to keep Sameer-Naina special friendship secret from the family. The makers do not wish to reveal Naina's love at this point. That is the only possible explanation.
But, I don't get this either. Preeti could have been Naina's best confidante at home just like Swati was at school for Naina. Naina didn't need to knit sweaters in the bathroom the whole time if she had Preeti by her side. It would have been lot more easier for Naina if Preeti would have known. Since, Preeti and Naina are so close, the makers should have got Preeti to know about this special friendship. It would be lot more exciting and more realistic.
The number of times Naina has blabbered Sameer's name in front of Preeti and Preeti still didn't get any clue đ