The CERN Hostel on the Meyrin site has rooms which can be booked by participants of the school. Please remember to look at the Booking Rules - CERN Hotel Terms and Conditions of Reservation | SCE before contacting them. In the webform the hostel management will ask you who your guarantor is, please use Kristina Gunne (kristina.gunne@cern.ch) and tick "official visitor" at the question of your status.
If you decide to stay outside of CERN, we suggest you to pick a hotel close to the tram line nr 18 ( direction CERN) which stops outside the CERN reception (building 33) http://home.web.cern.ch/directions.
We can recommend a few hotels on the Swiss side, close to the railway station where tram 18 leaves from:
Hotel Bernina Geneva - Bernina Geneva (bernina-geneve.ch)
Hotel Suisse - Hôtel Suisse (hotel-suisse.ch)
or closer to CERN and the airport, but still close to tram nr 18:
B&B Hotel Geneva Airport (New Hotel, good prices)
Overview (marriott.com) Geneva Marriott Hotel (Completely new)
Or
Holiday Inn Express Geneva Airport Hotel by IHG
The Cointrin airport is a taxi journey from CERN (around 35 CHF) and the cost of a tram ticket is 3 CHF.