Wow Petrova - as an identical twin you'd have had your own cs book! How's this...
You and your twin would are going to be separated because of your bad behaviour(?) but the dotty aunt who was meant to be getting your school things together/packing trunks etc (whilst your parents went off to India to do... errrr... something) would mislay letters/let you do your own shopping etc so secretly you could both go to the chalet school. You twins would also write to the other school, pretending to be your parents, saying you'd changed your mind and cancelling the place.
You'd then tell the aunt you'd make your own way to the train so she didn't have to get up early and she'd think you were being sweet so give you a large tip - which would pay for other twin's train ticket (your only worry was how to afford this, as parents bought you both train tickets to different schools before leaving). Once at the Chalet school you'd have had to take turns hiding (attic/boat house?) so no-one suspected there were secretly 2 of you and only wondered at your slightly odd personality changes and forgetfulness from 1 day to the next.
Sadly keeping each other informed of what you'd been up to would necessitate sleepless nights and Matey would notice you looking pale & heavy eyed and put you to sleep in the san with a 'dose' from Jack to sort you out.
Other twin would not know this & turn up to lessons next day - Matey would see other twin and march her to the san, only to find... ta-dah! twins.
Luckily Miss Annersley (after making you feel like very silly little girls for causing so much trouble) would secretly have disapproved of your being separated from your twin and (after a word with Miss Wilson) would petition your parents in India... probably your parents would meet up with Dick and Mollie Bettany so would hear how great the Chalet school is too & so you'd both be allowed to stay!
Of course the girls would be told never to speak of it to you, but would secretly think you were v cool. :)
Lol, can you tell I definitely wanted to be a cs girl - even though I was also rubbish at languages. :)