Zurich to Bern is a simple one-hour train ride. To get to Adelboden, you will take a train from Bern to Frutigen, and a Postbus from there. You will need reservations on the Postbus for your group but these may come with the group travel arrangements. You can contact SBB at the website Sam gave you to arrange for group fares.
From Adelboden to Venice! you will start by returning to Frutigen on the Postbus. From there you travel south to Brig, then Brig to Milan, and Milan to Venice. There is a direct train each day from Brig to Venice with no changes, which would simplify the journey and the booking process as you could do it all through SBB. It departs Brig at 9:44 am; to catch that you would start with a Postbus from Adelboden to Frutigen around 7:50 am. You could check the SBB website ( SBB.ch or rail.ch for English) to see if there is a later direct train. If not, and the 7:50 departure is not realistic, you can change trains in Milan and book Milan to Venice on the Italian website, trenitalia.com. I believe they also offer group discounts.
As for what to do in Venice, the whole place is a work of art and so different from anything they have seen before, the girls ( and you) will just enjoy exploring. We might be able to come up with a list of interesting sights which you could turn into a "treasure hunt." The basilica of San Marco is a must; especially if you include a visit to the balcony with the replicas of the much-traveled horses (the originals are just inside). They might also enjoy the tour of the doge's palace, especially the Secret itineraries tour ( must be booked in advance). Other interesting things for teens would be the Rialto Bridge and nearby fish and produce market, the Accademia bridge, the Dorsoduro waterfront ( Zattere), the small back streets of San Polo with nice little shops, gelato anywhere, the views from Punta del Dogana, the ghetto, a walk down Strada Nuova, a ride down the Grand Canal on the vaporetto. . . That is just a start. If you have runners or swimmers in your group there are some nice running routes ( only in early morning) and two very nice public pools. And beautiful churches ( my favorite is Santa Maria dei Miracoli) and amazing buildings everywhere.
Try to get them to learn a bit of the history of Venice and its place in the world so they will appreciate what they are seeing. It may be full of tourists today, but at one time Venice was the head of a powerful merchant empire based on supremacy of the seas. You might also encourage them to watch a movie filmed in Venice. Our favorite is Bread and Tulips, a light comedy, but the story involves a woman who ( to everyone's approval) leaves her philandering husband to stay in Venice where she found happiness. Summertime is a classic, with Katherine Hepburn, but may be too dated for teens. Maybe they would like The Tourist, a thriller/comedy with Johnny Depp and Angelina Jolie. Then you could go for dinner at the moderately priced restaurant/pizzeria Rosa Rossa, which Johnny Depp frequented during the filming in Venice. He signed the ceiling and left a jacket, which is now displayed in a glass case, the subject of many photos if you happen to end up in that room.
There was another Girl Scout group that got help here with a trip to Our Chalet and Italy. You could use the search function to find posts by Felicia, and her final trip report.
